DICTIONARY OF
PHILOSOPHY AND PSYCHOLOGY

Brain [AS. bregen]: Ger. Gehirn, Hirn (chiefly in compounds); Fr. cerveau encéphale; Ital. encefalo, cervello (in broad sense). That part of the central or axial nervous system which (in higher vertebrates) is enclosed within the skull; the expanded cephalic portion of the neural tube, including the centres or origin of the twelve cranial nerves, when present.

[This article consists of the following paragraphs: I. Historical; II. Embryology; III. Anatomy; IV. Comparative Anatomy; and V. Functions. A full bibliography of general works follows, and a glossary is added supplying brief definitions of more important organs.]

I. HISTORICAL. -- That the brain is in some way connected with thought was recognized very early, and this view prevailed among the Greek physicians in spite of the fact that Aristotle described the brain as the most bloodless and inert organ of the body. The Pythagoreans, Hippocrates, and Plato clearly recognized the head as the seat of the intellect and will. Descartes was the first to elaborate a consistent theory of brain functioning (cf. LOCALIZATION).

During the 17th century great advances were made in the purely anatomical study of the brain, yet Leuwenhoeck was the first to employ the microscope in its investigation. Reil introduced hardening and preservative processes. Rolando and Stilling may be said to have founded the new technique by introducing a method of preparing sections, though this process would have been relatively fruitless but for Gerlach's discovery of the possibility of staining the sections (cf. NEUROLOGY). Experimental neurology dates from Flourens, 1824; but its fruition began in 1870 with Fritsch and Hitzig.

II. EMBRYOLOGY. -- [In this and the following sections free use should be made of the glossary at the close of the article.] The brain, in common with the entire nervous and sensory apparatus, arises from modifications of the ectoderm. In higher animals the central nervous system appears as a plate extending down the dorsal axis of the ectoderm, and it is very early supplemented by accessory cephalic plates or bands composed of cells which are to be employed in the elaboration of the higher organs of special sense. The first stage in differentiation is the appearance of the medullary groove, which soon deepens and becomes closed dorsally to form the neural or medullary tube, out of whose substance the entire central nervous system is plastically constructed. At or near the suture formed by the closing of the neural tube arise the rudiments (Anlagen) of the sensory ganglia. Cf. NERVOUS SYSTEM (Histogenesis), and Plate A (BRAIN).

The tube becomes inflated at three points toward the cephalic extremity, thus forming three embryonic vesicles representing the primary fore-brain, mid-brain, and hind-brain, respectively.

The cephalic vesicle soon evaginates on either side, forming pouches (the primary optic vesicles) which project towards the skin, where they meet the in-growing rudiments of the lenses. In the course of their being moulded about the lens the original cavity is obliterated by the invagination of the lateral aspects forming double walls, or the retina and pigment layer of the choroid respectively. The cephalic portion of the primary fore-brain also expands, and, in all but the lowest vertebrates, divides into two hemispheres, whose ventricles remain connected by a median portion (the aula) via two apertures (the portae).

The second or mid-brain vesicle remains undivided, its roof forming the optic tecta in which are situated the primary centres for vision. From the dorsal aspect of the roof arise the four eminences, collectively constituting the corpora quadrigemina. The base of this segment is largely occupied by bands of fibres connecting higher regions with the medulla oblongata and cord, and constitutes the crus. The cavity of the mid-brain (mesencephalon) is gradually almost obliterated (in mammals), leaving only the aqueduct of Sylvius.

The third vesicle subdivides to form the cephalic (metencephalic) and caudal (myelencephalic) portion, the former being bounded by the pons and isthmus region and cerebellum, and the latter by the walls of the medulla oblongata proper. In this way the five most important divisions of the brain are marked out.

Scarcely less important than the vesicles in the architectonic of the brain are the flexures. The cephalic vesicle is flexed ventrad at an early stage, making room for the dorsal growth of the hemispheres and producing the cranial flexure. As though in compensation, the pons region is also flexed ventrad, affording room for the cerebellum, and, at a still later period, the cervical flexure marks the lower level of the medulla. By such means the axis of the brain is crumpled to accommodate the inequalities of growth.

As one result of these distortions, the actual cephalic terminus of the neural tube is obscure. Comparative researches indicate that the tube is, at an early stage, connected with the surface by a neuropore, whose position is supposed to be in some point of the lamina terminalis. See Plate A (NERVOUS SYSTEM).

The two hemispheres are probably discrete from the first, being separate evaginations from the primary fore-brain vesicle. In some cases they are represented simply by solid thickenings of the walls of the vesicle. In other cases the roof of the two hemispheres expands to form the mantle or pallium, which is at first membranous, and this, in turn, produces the dorsal walls of the cerebral hemispheres and their cortex, as well as a vascular membrane which, under the name of paraplexus and preplexus (plexus choroideus anterior), is folded into the cavities.

From this secondary fore-brain (prosencephalon) an evagination is thrust out from the ventro-cephalic extremity of each hemisphere to form the olfactory bulbs (containing the rhinocoele), and, after receiving accretions from the olfactory mucous membrane, these form the first central stations of the olfactory nerves. These, and the stalk connecting them with the hemispheres (crura), and the eminence in which they are inserted (lobi olf.), together with the entire median and ventral parts of the cerebrum, including the hippocampus and part of the gyrus fornicatus of higher brains, have been grouped together as rhinencephalon, from their association with the olfactory function.

The median unaltered part of the primary fore-brain becomes the diencephalon, including the thalamus and its appendages. The cavity is reduced to a narrow slit (the third ventricle or diacoele). This extends ventrally into the infundibulum and optic recesses.

Within the walls of the diencephalon there arise a variety of nuclei of great importance in the physiology of the brain. Of these the mammillaria are associated with smell, the geniculata with vision, and the subthalmicus, and perhaps the ruber, with general and somatic sensation.

The second embryonic vesicle is less profoundly modified, though its ventral aspect is largely reinforced by fibres to and from the higher regions, thus forming the pes pedunculi.

The metencephalon and myelencephalon are formed from the walls of the third vesicle, which collectively constitute the rhombencephalon. The roof of the entire region is primarily membranous (tela-form), and so remains in some lower animals. It is partly transformed into a complicated plexiform organ which, with outgrowths of the pia, constitutes the metaplexus. In various portions of the rhombencephalon lateral outgrowths of cellular matter encroach upon the roof, forming more or less complete roofs over the fourth ventricle. The most constant of these outgrowths is the cerebellum or epencephalon. The cellular materials of which the cerebellum is largely composed arise from rudiments in the lateral walls of the tube, and are pushed dorsad, forming two lateral hemispheres which coalesce dorsally, though a centre of proliferation also exists at the caudal extremity. See NERVOUS SYSTEM (Histogenesis).

A special group of nuclei (n. dentatum, &c.) is developed in the ventral aspects of the cerebellum by proliferations of ventricular epithelium, and similar bodies (the olives) are formed by diverticles of ventricular epithelium in the base of the medulla oblongata. The floor of the medulla, otherwise, is largely occupied with fibres from the myelon and the root nuclei of the cranial nerves, whose development obeys similar laws to those of the cord.

The entire length of the brain has been laid off into embryonic zones, each with its intrinsic peculiarities. (1) The basal plate, occupying the ventral median line; (2) laterally the fundamental plates, containing the root zones of the motor nerves; (3) alar plates, with the root zones of the sensory nerves; (4) dorsal plate forming the roof. (1) and (4) are primarily membranous and, as the most conservative portions of the nerve-tube, are especially valuable in determining relationships. Cf. NERVOUS SYSTEM (Comparative Anatomy).

III. ANATOMY. Recalling the fact that all the structures of the adult brain are derived from plicated and thickened modifications of the medullary tube, the essential relations are best seen in a hemisection of the entire brain, especially that of an embryo. See Fig. 2; cf. also Plate A (NERVOUS SYSTEM).

In the adult the front of the original forebrain (lamina terminalis) occupies a position well toward the middle of the brain. Ventrally it terminates in the recessus preopticus, cephalad of the chiasm.

The two great prosencephalic commissures originally lie in the lamina terminalis, though this relation is obscured in adult life. Of these commissures the ventral one (precommisure) connects the ventral portion of the prosencephalon, while the dorsal one (callosum) is the commissure of the pallium proper. The great arch which this commissure makes over the portae and aula leaves a space closed only by the thin septum lucidum. The membranous part of the roof next following is the invaginated vascular preplexus; then the evaginated preparaphysis and the velum, finally the post-paraphysis (Zirbelpolster) and the supra-commissure.

In the higher brains the mesal and caudal part of the hemispheres, viz. the hippocampal region with its olfactory centres, is connected with its fellow of the opposite side and the base of the thalamus by a transverse band (the hippocampal or fornix commissure). In man the fornix is appressed upon the callosum lying ventrad of it, but leaving a triangular interval, the lyra Davidis. The corpus fornicis receives fibre bands (crura) from the hippocampus, and sends others (columnae) towards the mammillaries.

The roof behind the supra-commissure is produced dorsad to form an organ of variable size and structure which, in lower animals, stands in intimate relation with a median PARIETAL ORGAN (q.v.). This epiphysis (sensu stricto) is vascular, membranous, or absorbed in various groups. Near the point where the epiphysis unites with the roof of the thalamus are the cellular habenulae with their commissure. Then follows (passing caudad) the short lamina intercalaris and the post-commissure at the cephalic limit of the mesencephalon.

The ventral aspect of the thalamus exhibits the post-optic recess, infundibulum, and mammillary recess; while a vascular outgrowth from a point caudad of the infundibulum occurs in various aquatic animals, and constitutes the saccus vasculosus. These are all diverticula of the third ventricule. The mammillaries are cellular masses caudad of the tuber cinereum (see Glossary), and are end-stations for the fornix fibre columns. The ventral part of the thalamus, or hypothalamus, contains at least four commissures beside the chiasm of the optic nerves.

The floor of the mesencephalon is composed of the massive pes pedunculi, a strong tract of motor fibres from the cortex to the pyramidal tracts of the spinal cord. This is the chief efferent path from the brain to the trunk. Lying dorsally of it is the tegmentum. It contains the afferent nuclei (ruber and niger), whose relations seem to be with the motor systems especially, serving as shunting stations for the cerebellum. The tegmentum also contains the lemniscus fibres, which are the great conductors of sensory impressions to the brain. They associate the sensory nerve roots of the medulla oblongata and cord with the caudal portion of the mesencephalon roof. The dorsal walls of the mesencephalon contain optic centres (tectum, &c.), as well as stations for the communication with the co-ordinating motor nuclei; and, accordingly, in the caudal portions lie the nuclei of the third and fourth nerves.

The medulla oblongata and cerebellum constitute a single region (rhombencephalon). The medulla oblongata is that part of the brain which forms the transition to the spinal cord, and, accordingly, it departs less than the rest of the brain from the type of structure exhibited by it. The embryonic third vesicle is greatly enlarged to form the metacoele, and its roof is largely membranous. In the massive part of the embryonic medulla oblongata there may be distinguished a basal plate, two fundamental plates, and two alae. It is by complications and proliferations from the epithelium of the latter that the lateral and dorsal outgrowths, including the olives (His) and the cerebellum (Herrick), are developed.

Ventrally the surface exhibits the ventral columns and pyramids which are crossed transversely by the pons fibres from the middle cerebellar peduncle and the trapezoides fibres from the roots of the eighth nerve. Dorsally, after removing the cerebellum, the dorsal columns (funiculi gracilis et cuneatus) of each side unite to form the restiformia, and these pass into the post-peduncles of the cerebellum. The dorsal fibres of the cord (Goll's and Burdach's columns) terminate in the nuclei of the funiculi gracilis et cuneatus, and are continued cephalad into the lemniscus by bundles arising in these cell clusters. These soon decussate and take their place in the ventro-lateral region laterally from the pyramid fibres, which also cross at this level. This more or less continuous decussation in the medial line forms the raphe.

We may follow this general review by a more detailed description of the major divisions.

Cerebrum. In the development, after separation of the hemispheres, a thickening in the base of each projects into the ventricle and constitutes the striatum, an intermediary station through which pass nearly all the fibres connecting the cortex with lower parts of the system. The striatum is divided by the fibres of the corona radiata or internal capsule into the nucleus lentiformis and the n. caudatus, each with subordinate portions. As the development proceeds the hemispheres become flexed about an axis terminating in the fossa Sylvii of either side, and the result of this and analogous alterations is expressed by dividing the external aspect into lobes (l. frontalis, parietalis, occipitalis, temporalis, insula, &c.). The internal concomitant of this process is expressed by the subdivision of the lateral ventricles into the cephalic, caudal, and ventral cornua. At the same time the rapid expansion of the surface, due to formation of the cortex, results in irregularities of the cortex constituting gyri or convolutions separated by fissures or sulci. The most mesal and caudal portion of the cortex is strongly conduplicated, forming a reverse curve in section, and giving rise to the hippocampal region or cornu Ammonis, with the gyrus hippocampi, whose section is S-shaped, and the gyrus uncinatus, which is folded in the reverse sense into the inner limb of the S. The fibres collected on the ventricular aspect of this region form the alveus, while the tangential ectal fibres pass through the fimbria into the fornix. See Fig. 4.

The transection in Plate B (BRAIN), Fig. 1 conveys a good idea of the general relations. The topography of the cortex may be gathered from Figs. 5, 6, and 7. Cf. LOCALIZATION (cerebral).

The superficial layer of grey matter of the cerebrum, containing the highest centres for sensation and voluntary motion (cortex in strict sense), is usually regarded also as the seat of consciousness. Ontogenetically the cortex is derived from the pallium, and includes the outer cellular part as contrasted with the inner fibrous parts of the hemisphere's walls. In the ventral and mesal regions the cortical layers merge into ventricular grey matter, with which they have a common origin from the proliferating ventricular epithelium.

Although the arrangement and distribution of cortical elements varies in different regions, they may be conveniently included in four layers outside of the white matter. The most superficial zone is sparsely provided with cells in which ramify the dendrites of the deeper nerve cells. In this layer also originate the tangential fibres connecting with other regions. This is the tangential fibre zone (also molecular, neuroglia, or Cajal-cell layer). The second layer is filled with small pyramidal cells, which tend to increase in size in deeper portions, forming a transition to the third later of large pyramids. Beneath the latter is a zone of variously placed polymorphic cells.

The relations between the cells of these layers are seen in Plate B (BRAIN), Fig. 2. The pyramidal cells give rise to strong neurites, which extend into projection-tract fibres and which occasionally give off strong collaterals, passing via the callosum to homologous regions of the opposite hemisphere. Other cells send their neurites into the tangential zone outward: see ASSOCIATION FIBRES.

The framework of the cortex is originally supplied by the spongioblasts of the endyma (see NERVOUS SYSTEM, Histogenesis), and, in the adults of the lower animals, these cells continue to span the entire thickness of the cortex. In the massive cerebrum of higher animals the so-called neuroglia cells supplement the primitive spongioblastic framework.

The neurites of the cortical cells destined to the lower centres, collecting in the corona radiata, and afterwards constituting the internal capsule, are there segregated somewhat in accordance with their sources; and, accordingly, various parts of the capsule contain fibres with different functions. See LOCALIZATION.

The speech centre (Broca's region) contributes a special tract to the pyramids. The parietal motor zones are also represented by more or less distinct bands in the internal capsule. The optic radiations connect the optic centres of the thalamus with the occipital cortex, and it is probable that each sensory area has its reflex tract connecting with the appropriate lower centres. Special tracts from the striatum pass to the tegmental nuclei.

Cerebellum. Although derived from outgrowths of the lateral walls of the metacoele, the cerebellum in mammals consists of a median vermis and two lateral hemispheres, with two or more smaller paired bodies (flocculi). The surface is marked by convolutions and depressions analogous to the gyri and fissures of the cerebrum. The cortex of the cerebellum (Fig. 8) is extraordinarily complex, and beautifully illustrates the apparatus for nervous discharge by contiguity rather than by structure continuity. Three layers may be distinguished above the white fibre zone. Of these the middle layer, composed of the cells of Purkinje, is most important. These large pyramidal cells are in a single layer, and their very numerous dendrites ramify in mazy arborizations within the ectal or molecular layer (Fig. 9). The neurites from the bases of the cells of Purkinje pass in regular order into the ental white layer, but give off, in their course, numerous collaterals which enter into close relations with other similar cells. In the ectal layer, or zona molecularis, the dendrites of the Purkinje cells are intimately associated with the terminal ramifications of the scandent fibres, which, arising in unknown (probably extra-cerebellar) cells, ascend between the Purkinje cells and ramify among their dendrites. Other cells within the molecular layer give off tangential fibres, from which arise collaterals which descend and surround with their fine ramifications the bodies of the Purkinje cells, thus effecting a co-ordination with the latter. Beneath the Purkinje layer is the zona granulosa, composed essentially of small polygonal cells having large nuclei whose dendrites soon subdivide, while their neurites enter into the molecular layer and there bifurcate. Other cells of the grandular layer resemble the cells of Purkinje, but their neurites ramify in the same layer.

The external connections of these cells are problematical. Of the three pairs of cerebellar peduncles, the cephalic pair (superior brachia) connect the cerebellum with the tegmental nuclei and with Gower's ascending tract of the spinal cord. The middle peduncles consist, chiefly at least, of fibres from the pons, which pass to the lateral cerebellar hemispheres (pilea). These fibres are in part neurites from the Purkinje cells of the cerebellar cortex, terminating about the cells of the nuclei pontis of the same and the opposite side, and also going an unknown distance cephalad in the pyramids; in part they are neurites of the cells of the nuclei pontis, terminating probably as the scandent fibres of the molecular zone of the cerebellar cortex. The cells of the nuclei pontis are in communication not only with the terminal arborizations of the Purkinje cells, but with collaterals from the underlying pyramidal fibres. We clearly have here provision for both ascending and descending fibres connecting the cerebellar cortex with the great motor and sensory tracts leading down from the cerebral cortex. The caudal peduncles enter the corpora restiformia and contain fibres to the fleece and vermis. They receive the ascending direct cerebellar tract of the spinal cord, fibres from the great lemniscus system (nuclei funiculi gracilis et cuneatus), and, directly or indirectly, fibres from the auditory nerve, and others less thoroughly known.

The Membranes (meninges). The membranous envelopes of the central nervous system are three in number, the innermost or pia belonging strictly to the nervous organ, while the outer one, the dura, is more closely associated with the bony walls. The middle membrane, or arachnoid, is variable in its relations. Sometimes the outer envelope is called pachymenix, and the inner leptomenix, though these terms appear more often in combinations, such as pachymeningitis. The dura within the cranium is apparently simple, but within the spinal cavity it is cleft into an inner layer, or theca, and an outer one closely attached to the vertebrae. The interval between these is the epidural space. The space between the cerebellum and cerebrum is occupied by a fold of dura (sometimes ossified) constituting the tentorium. >From the middle of the tentorium a similar fold extends between the two cerebral hemispheres dorsally of the callosum, forming the falx and enclosing the great longitudinal blood sinus. The membranes are subject to various diseases, often of a serious character. Leptomeningitis is a purulent inflammation of the pia and adjacent parts of the brain. It is an infectious and often epidemic disease due to a diplococcus. Primary meningitis originates at the base of the brain; minute hemorrhages in the brain substance and distension of the ventricles are frequent concomitants. The prognosis is grave, and recovery in any event usually but partial. Tubercular meningitis is apparently due to infection from other organs. Pachymeningitis is an inflammation of the dura, and is an accompaniment of various diseases, especially chronic diseases of the brain itself. The meninges are freely supplied with sensory cerebro-spinal and vaso-motor sympathetic nerves.

Circulation in the Brain. The circulation within the brain is chiefly provided for by the vessels of the meninges, and those of the intrusions of the brain walls forming apparently intra-ventricular plexuses. Although Mendel has found that the pressure in the cortical arteries is less than that in the carotids, it is true that any serious alteration in the vascular pressure is attended with functional disturbance. The intra-cranial arteries fall into two systems. (1) The meningeal, or dural, which ramify upon the dura and diploi of the cranium. Nearly the whole dura is supplied by branches of the external carotid, of which the medi-dural is the largest. The internal carotid supplies the hypophysis and Gasser's ganglion. The vertebral gives rise to the post-dural, which supplies the dura of the postcranial fossa, especially the falx. (2) Intrinsic arteries are such as supply the brain substance. Four arteries supply this system, the paired carotids and the vertebrals. These primary vessels combine to form the circle of Willis in the brain base, and from this polygonal vessel arise all the cerebral arteries. Intra-vascular pressure is less in the grey than in the white matter. The veins likewise are divided into dural and intrinsic veins, and all of the dural and most of the intrinsic veins open into sinuses or spaces with non-collapsible walls. The largest sinus is the longitudinal, parallel to the sagitall suture, and is the vessel presenting most difficulty to intra-cranial operation. The lymphatic system of the brain, though obscure, is not wholly absent. Its most rudimentary form is seen in the peri-cellular spaces. The lymph canals collect and pass out in company with the veins. The endyma and hypophysis are closely associated with the lymph-vascular systems.

IV. FUNCTIONS OF THE BRAIN. Two general theories of brain function may be distinguished. According to the earlier, more mechanical theory, the brain is a collection of different organs which, while more or less connected serially by nervous paths, are nevertheless distinct in nature and function. This anatomical survey of the psychological concept of discrete faculties received strong support from the data of localization, although there have always been advocates of the doctrine of cerebral unity. Modern histological researches have reinforced the argument for the essential unity of the brain by showing that not only are all parts derived from the homogeneous brain-tube, but that the cellular elements have a homologous origin from the ventricular epithelium. Furthermore, the discovery of communication by contiguity rather than actual continuity of structure, indicates that the intercommunication may be much more complicated and extensive than was formerly supposed possible. The vast complexity of the fibrous processes from the various cells, and the systems of collaterals and association fibres, reveal in the brain a degree of solidarity inconsistent with a rigid localization. The conditions seen in brains of Amphibia, where an almost undifferentiated zone of cells adjacent to the ventricles serves for all the various and diverse functions, and where chains of associated cells mark the course of tracts of the highest importance, forcibly impress the idea that it is rather the relations than the structure of the cells which determine the function, and that the nervous or physical resultant is, in a sense, the algebraic sum of the activities -- an equation of function.

The neurocytes may be roughly classed as centripetal, centrifugal, and intercalary, with respect to the central organ or stations; though, strickly speaking, each neurocyte is simply a node in a path, the cellipetal parts being dendrites; the cellifugal, the neurite. The third class seems to serve to divide a tract or to switch stimuli from the direct to an indirect route, as in the tegmental nuclei. See Figs. 9, 10, 11.

There is much reason to believe that within, as well as outside the central system, many neurocytes may be merged in a single long circuit or tract. In some cases the cell body is thrown out of circuit and remains a mere appendage with a nutritive function only, while in others the impulse passes through the cell, and is there probably reinforced or modified. Good illustrations of the complicated paths pursued by a stimulus are afforded by the special senses. For example, an irritation in the nasal epithelium passes via a neurite, or from cell to cell to a neurite whose fine arborizations enter a glomerule and communicate with one of the dendrites of one of the mitral cells of the pero. See Fig. 6 (NERVOUS SYSTEM). Thence the stimulus may pass via a neurite or chain of cells to the hippocampus, and there a similar communication is effected with the dendrites of the long pyramids of the hippocampal cortex, in the bodies of which we may suppose an excitation is established, the effect of which upon the total equilibrium of the sensorium constitutes a sensation of smell. But, at the same time, other neurites convey stimuli via the fimbria and fornix to the thalamus, where reflex associations of the most complicated sort are effected. In fact the whole infra-cortical tone is perhaps modified, and profound changes in circulation and the relative receptivity of various sense organs may result. Nor are these sub-conscious changes devoid of their secondary influence on consciousness.

The theory of consciousness which seems best to conform to the conditions of brain structure and its observed unity is that each conscious state is an expression of the total equilibrium of the conscious mechanism, and that intercurrent stimuli are continually shifting the equilibrium from one to another class of activities. In other words, the sensation accompanying a given colour presentation is not due to the vibrations in the visual centre in the occipital lobe, but to the state of cortical equilibrium or the equation of cortical excitement when that colour stimulus predominates. Previous vestigal excitements and co-ordinations with the data from other cortical centres all enter into the conscious presentation. As the wave of excitation passes from the visual centre to other parts, the proportional participation of other centres increases, producing a composite containing more distantly related elements.

What the exact nature of the infra-conscious processes in the thalamus, &c., is, may never become wholly clear. The stimuli from the organ of sense are said to be organized for presentation to the cortical centres. A very important function also is the proper provision for the complex reflex co-ordinations. Removal of any centre or area impairs the equilibrium, and so, aside from suppressing the function of the destroyed organ, alters the 'tone' of the consciousness.

The existence of non-functioning cells or portions from which new neurocytes may develop on occasion, is rendered probable by the data of histogenesis and by the occurrence of functional substitution.

That the cerebellum does not participate in the psychical, emotional, or generative functions has been made probable by recent experiments. That it has no direct relation to the fifth or eighth nerve roots seems likely, although some connection with the vestibular apparatus is conceded.

The direct effect of injury to or disease of the entire cerebellum is disturbance of motor equilibrium and co-ordination evidenced by imperfect station and locomotion without notable reduction of motor power. Astasia, or unsteadiness of head, trunk, and limbs, is permanent. Injury confined to the median lobe produces similar symptoms, but less pronounced, while unilateral injury affects the equilibration of the same side. Section of the peduncles of one side produces essentially the same effects as destruction of the corresponding lateral lobe of the cerebellum. Section of the middle peduncles results in atrophy of the nucleus pontis of the opposite side and the connecting pons fibres. There is evidence that removal of one half of the cerebellum increases the excitability of the opposite cerebral hemisphere.

The medulla oblongata participates largely in the reflex functions of the spinal cord, but is pre-eminently the locus of the primary centres of special sense. Here, too, the centres of the vagus nerve provide for vital co-ordinations. The term 'vital node,' though perhaps an anatomical fiction, expresses the intimate co-ordinations between the centres for vascular, respiratory, and digestive functions in centres immediately adjacent to the floor of the metacoele. The psychical importance of these centres lies in their intimate, though obscure, relation to the emotions. The anatomical basis for this connection is, as yet wholly unknown.

For discussion of the phylogeny of the Brain, see NERVOUS SYSTEM.

V. COMPARATIVE ANATOMY. -- Comparisons of the human brain with that of brutes are made in various ways, the relations of mere volume or weight being untrustworthy. In general, it is obvious that the extent to which consciousness has been interpolated in the cycle of nervous processes increases as we ascend the scale. The area of cortex devoted to the immediate spheres of the sense organs is surprisingly great in lower vertebrates. The lowest of the special senses (smell) is found to occupy a relatively enormous proportion of the cortex in reptilia, and even in marsupials such a disproportion is conspicuous. It is only in higher mammals that a large proportion of the cortex seems to have been reserved for correlations of higher orders. The apparent absence of a callosum in marsupials, &c., has a morphological rather than functional bearing, for the associational fibres proper to the callosum pass via the precommissure. In the estimation of the position of a brain type in the scale, use is made of certain indices. The occipital and parietal indices are the distances along the mesal margin of the hemispheres intercepted by the intersection of the external parieto-occipital fissure and the central Rolandic fissure respectively, measuring from the occipital pole in the first case, and from the occipito-parietal intersection in the other; the measurements being in hundredths of the entire length along the dorsal margin of the hemisphere. A table of such measurements is given as follows: --

In low apes there is an enormous increase in the occipital portion of the border, whereas in high apes the amount of increase is smaller. Only Primates, which alone possess well-developed occipital lobes, exhibit transitory fissures of the cerebrum. The cuneus in certain abnormal conditions of the human brain resembles that of apes.

The insula is larger in the male than in the female, and in the European than in lower races. In the relations of the insula and its opercula the embryonic conditions in man resemble those of apes. Cf. INDEX (cephalic), and ANTHROPOID.

Literature: Excellent literature lists will be found in SCHWALBE'S Neurologie, in QUAIN'S Anatomy, in Amer. Textbook of Physiol. (Philadelphia, 1896), and in most of the other general works cited below. EDINGER'S Berichte published in Schmidt's Jahrb. and MERKEL und BONNET'S Ergebnisse give comprehensive annual summaries. L. F. BARKER, The Nerv. Syst. and its Constituent Neurones (New York, 1899; 1122 pp., 683 figs.), is an excellent and comprehensive digest of the literature, especially the most recent, on nerve-cells and their relations.

H. C. BASTIAN, The Brain as an Organ of Mind (London, 1880); Über das specifische Gewicht des Gehirns, Arch. f. Heilk. (1886). W. V. BECHTEREW, Die Leitungsbahnen im Gehirn und Rückenmark (Leipzig, 1884). L. BIANCHI, Functions of the Frontal Lobes, Brain, xviii (1895). TH. L. W. V. BISCHOFF, Das Hirngewicht des Menschen (1880). E. BRISSAUD, Anatomie du cerveau de l'homme (Paris, 1893). A. BROCA, Anatomie descriptive des circonvolutions cérébrales, Gaz. hebdom., xxxviii (1891). T. E. CLARK, Comparative Anatomy of the Insula, J. of Compar. Neurol., vi (1896). D. J. CUNNINGHAM, The Complete Fissures of the Human Cerebrum and their Significance in connection with the Growth of the Hemisphere, J. of Anat. and Physiol., xxiv (1890); The Surface Anatomy of the Cerebral Hemispheres, Cunningham Mem. of the Roy. Irish Soc., No. vii (1892). B. DAVIS, Contributions toward determining the Weight of the Brain in different Races of Men, Philos. Trans. (1869). C. DEBIERRE, Sur les anomalies des circonvolutions du cerveau de l'homme, C. R. Soc. de Biol., iii (1891). DÉJERINE, Anatomie des centres nerveux, i (Paris, 1895). H. H. DONALDSON, Anatomical Observations on the Brain and Sense Organs of the Blind Deaf-mute Laura Bridgman, Amer. J. of Psychol., iii, iv (1890-1); The Growth of the Brain (London and New York, 1895). T. DWIGHT, Remarks on the Brain of a Distinguished Man, Proc. Amer. Acad. Sci., xiii (1887). L. EDINGER, Vorlesungen über die nervösen Centralorgane des Menschen und der Thiere, 5th ed. (1896, trans., 1899). EXNER, Zur Kenntniss vom feineren Baue der Grosshirnrinde, Wien. Sitzungsb., III Abth., 1xxxiii (1881). FERRIER, The Functions of the Brain, 2nd ed. (London, 1886). E. FLATAU, Atlas of the Human Brain and Description of the Course of the Nerve Fibres, trans. by Nathan and Carslaw (Glasgow, 1894). P. FLECHSIG, Die Leitungsbahnen im Gehirne und Rückenmarke des Menschen (Leipzig, 1876); Gehirn und Seele: Rectoratsrede (Leipzig, 1894). M. FOSTER, A Textbook of Physiol., 7th ed. (London and New York, 1891). GANSER, Gehirn des Maulwurfs, Morphol. Jahrb., vii (1882). A. V. GEHUCHTEN, Anatomie du système nerveux de l'homme, 2nd ed. (Louvain, 1897). C. GOLGI, Untersuchungen über den feineren Bau des centralen und peripherishchen Nervensystems, trans. from the Italian (Jena, 1894). B. V. GUDDEN, Gesammelte und hinterlassene Abhandlungen (Wiesbaden, 1889). D. J. HAMILTON, On the Corpus Callosum in the Adult Human Brain, J. of Anat. and Physiol., xix (1885); On the Corpus Callosum in the Embryo, Brain, viii (1885). H. HELD, Die centrale Gehörleitung, Arch. f. Anat. u. Physiol. -- Anat. Abth. (1893); other papers on the medulla and the quadrigemina, same periodical (1891-3), in Neurol. Centralbl. (1890), and in Abhandl. d. kgl. sächs. Gesell. d. Wiss. (1892). J. HENLE, Handbuch der Nervenlehre des Menschen, 2nd ed. (Braunschweig, 1879). HERMANN, Handb. d. Physiol. (1879). C. L. HERRICK, Illustrations of the Architectonic of the Cerebellum, J. of Compar. Neurol., i (1891), and The Histogenesis of the Cerebellum, ibid., v (1895); numerous other papers in the same journal (1891-9). HERVÉ, La circonvolution de Broca, Thèse (Paris, 1888). A. HILL, The Plan of the Central Nervous System (Cambridge, 1885). W. HIS, Zur allgemeinen Morphologie des Gehirns, Arch. f. Anat. u. Physiol. -- Anat. Abth. (1892). S. HONEGGER, Vergleichendanatomische Untersuchungen über den Fornix (Zürich, 1891). T. H. HUXLEY, On the Brain of Ateles paniscus, Proc. Zool. Soc. (1861). A. V. KÖLLIKER, Handb. d. Gewebelehre des Menschen, ii (Leipzig, 1896). W. KRAUSE, Über Gehirngewichte, Allg. Wien. med. Zeit. (1888); and Int. Monatssch. f. Anat. u. Physiol., v (1888). LANGLEY AND GRÜNBAUM, On the Degenerations resulting from Removal of the Cerebral Cortex and Corpora Stiata in the Dog, J. of Physiol., xi (189?). LUYS, Nouvelles recherches sur la structure du cerveau et l'agencement des fibres blanches cérébrales: L'Encéphale (1884). J. MARSHALL, Relations between the Weight of the Brain and its Parts and the Stature and Mass of the Body in Man, J. of Anat. and Physiol., xxvi (1892); The Brain of the late George Grote, with Comments and Observations on the Human Brain and its Parts generally, J. of Anat. and Physiol., xxvii (1892). TH. MEYNERT, Psychiatry, trans. by Sachs (1885); also art. Brain, in Stricker's Histol. (1872); Das Gesammtgewicht und die Teilgewichte des Gehirnes, &c., Vtljsch. f. Psychol. (1867). G. MINGAZZINI, Il Cervello in relazione con i Fenomeni psichici; Studio sulla Morfologia degli Emisferi cerebrali dell' Uomo (Turin, 1895). MONAKOW, several works bearing especially on the optic connections in Arch. f. Psychol. (1889, 1892, 1895). H. MUNK, Über den Hund ohne Grosshirn, Arch. f. Anat. u. Physiol. -- Physiol. Abth. (1894). H. OBERSTEINER, Anleitung beim Studium des Baues der nervösen Centralorgane im gesunden und kranken Zustande, 3rd ed. (Leipzig, 1896), Eng. trans. of earlier ed., Philadelphia (1890); Die neueren Forschungen auf dem Geblet des Centralnervensystems, kritisch beleuchtet, Wien. med. Wochen., xlv (1895). H. F. OSBORN, The Origin of the Corpus Callosum; a Contribution upon the Cerebral Commissures of the Vertebrata, Morphol. Jahrb., xii (1887). E. PUSATERI, Sulla fine Anatomia del Ponte di Varolio nell' Uomo, nota prelim., Riv. di Patol. Nerv. e Ment., i (1896). A. RAUBER, Lehrb. d. Anat. des Menschen, 4th ed., ii. Abth II (Leipzig, 1894). Reference Hand-Book of the Medical Sciences, Wm. Wood & Co., articles by Browning, Wilder, Spitzka, Baker, Herrick, and others. G. RETZIUS, Biol. Untersuch. (Stockholm, 1875-95); Atlas of the Brain. J. V. ROHON, Zur Anatomie der Hirnwindungen bei den Primaten (München, 1884). H. SACHS, Vorträge über den Bau und die Thätigkeit des Grosshirns (1894). E. A. SCHAEFER, Spinal Cord and Brain, iii, Part I of Quain's Anatomy, 10th ed. (London and New York, 1893). SCHIFF, Lehrb. d. Physiol. G. SCHWALBE, Lehrb. d. Neurol. (Erlangen, 1881). SHERRINGTON, Nerve Tracts degenerating secondarily to Lesions of the Cortex Cerebri, J. of Physiol., xi (1890); Further Note on Degenerations following Lesions of the Cerebral Cortex, J. of Physiol., xi (1890). O. SNELL, Die Abhängigkeit des Hirngewichtes von dem Körpergewicht und den geistigen Fähigkeiten, Arch. f. Psychiat., xxiii (1891); Das Gewicht des Gehirnes, Münch. med. Wochensch. (1892). E. C. SPITZKA, Architecture and Mechanism of the Brain, J. of Nerv. and Ment. Dis. (1878 f.). TOPINARD, Le Poids de l'Encéphale, Mém. Soc. de l'Anthropol., iii (1888). W. TURNER, The Convolutions of the Human Cerebrum (Edinburgh, 1866); The Convolutions of the Brain, Verh. d. int. Congresses (Berlin, 1890), ii, and J. of Anat. and Physiol., xxv (1890). VULPIAN, Leçons sur la physiologie du système nerveux (Paris, 1866). W. WALDEYER, Die Hirnwindungen des Menschen, Verh. d. int. Congresses (Berlin, 1890); Das Gibbon-Hirn, Virchow-Festschrift, iii (1891). MAX WEBER, Vorstudien über das Hirngewicht der Säugethiere, Festschrift f. Gegenbaur, iii (Leipzig, 1896). CARL WEIGERT, Beiträge zur Kenntniss der normalen menschlichen Neuroglia, Abh. d. Senckenberg'schen Naturf. -Ges. in Frankfurt a. M., xix. I (1895). C. WERNICKE, Lehrb. d. Gehirnkrankheiten, i (1881); Grundriss der Psychiatrie in klinischen Vorlesungen, Theil I, Psycho-physiologische Einleitung (Leipzig, 1894). B. G. WILDER, The Outer Cerebral Fissures of Mammalia, especially of the Carnivora, and the Limits of their Homologies, Proc. Amer. Assoc., xxii (1873); On the Removal and Preservation of the Human Brain, Trans. Amer. Neurol. Assoc. (1883); J. of Nerv. and Ment. Dis., N.S., viii (1883); Methods of Studying the Brain, The 'Cartwright Lectures' for 1884, N. Y. Med. J., xxxix-xl (1884); The Cerebral Fissures of Two Philosophers, Chauncey Wright and James Edward Oliver, J. of Compar. Neurol., v (1895) and J. of Nerv. and Ment. Dis., N.S., xx. (1895). W. WUNDT, Gehirn und Seele, in 'Essays' (Leipzig, 1885); Grundzüge d. physiol. Psychol. (4th ed.), Erster Bd., Erster Abs. (Leipzig, 1893).

[The cross-references in SMALL CAPITALS refer to other topics in the Dictionary, those in Roman type to other terms in this glossary.]

Afterbrain. The medulla oblongata.

Ala Cinerea. The thin dorsal wall of the metacoele where it joins the velum caudale.

Alar Plate. One of the regions of the medullary tube. The longitudinal zone on either side near the base from which arise the sensory nerve roots.

Alveus. The fibrous layer of the ventricular surface of the gyrus hippocampus.

Ammonshorn: see Hippocampus.

Ansa Lentiformis. Fibres from the putamen of the nucleus lentiformis which, instead of passing through the internal capsule, pass near the brain base on their way to the thalamus.

Ansa Peduncularis. Fibres forming an arch superficially to the cerebral peduncles and ventrad of the nucleus lentiformis; the 'substantia innominata.'

Aquaeductus Sylvii: see Iter.

Arachnoidea. The middle one of the three membranes investing the brain. See BRAIN (Meninges).

Arbor Vitae. The peculiar ramose appearance presented by a section of the cerebellum, which is due to the distribution of the white and grey matter.

Area Olfactoria. The region on the ventral aspect of the hemispheres through which the olfactory radiations pass on their way to the hippocampus.

Area Parolfactoria. The median aspect of the hemispheres cephalad of the lamina terminalis and ventrad of the free cortex [Edinger].

Area Post-perforata: see Perforata.

Area Preperforata: see Perforata.

Aula. The remnant of the first embryonic vesicle which connects the third ventricle by way of the portae with the lateral ventricles.

Axial Lobes. The solid masses of grey matter forming the massive part of the hemispheres in some low vertebrates. They contain the homologues of the striata and some additional undifferentiated structures.

Axis, Cerebro-spinal: see Neuraxis.

Basal Ganglia. The masses of grey matter below the cortex forming the terminal stations of the lower projection systems. The corpora striata [Edinger], or these bodies together with the nuclei of the thalamus [many authors].

Basal Plate. One of the regions of the medullary tube. The longitudinal zone forming the median ventral portion (primarily membranous).

Brachium Coniunctivum Cerebelli. The pre-peduncle. See Pedunculi Cerebelli.

Brachium Inferius Cerebelli. The post-peduncle. See Pedunculi Cerebelli.

Brachium Opticum. The central prolongation of the optic tracts, together with fibres from the cortex.

Brachium Pontis. The medi-peduncle. See Pedunculi Cerebelli.

Brachium Quadrigeminum: see Brachium Opticum.

Brachium Superius Cerebelli. The pre-peduncle, See Pedunculi Cerebelli.

Broca's Region. The cortical region near the Sylvian fissure associated with the function of speech.

Bulb. The medulla oblongata, especially in compounds, as bulbar paralysis; a term the use of which is not to be commended.

Bulbus Olfactorius. The protuberance from the hemisphere which receives the olfactory 'nerve' (more properly radix: see Radix Olfactorius). It consists of an external part, or pero, and an axial portion, the pes.

Burdach's Column. The more lateral of the two dorsal fibre columns of the cord. In the medulla it becomes the funiculus cuneatus.

Calamus Scriptorius. The most caudal tip of the fourth ventricle.

Calcar [Avis]. A protuberance into the caudal cornu of the lateral ventricle due to the invagination of the cortex in the region of the fissura calcarina; the hippocampus minor.

Canalis Centralis. The remnant of the cavity of the medullary tube of the embryo and the continuation within the spinal cord of the ventricles, or coelia, of the brain.

Capsules. The internal and external capsules are white bands between the nuclei lentiformis and caudatus (capsula interna) and ectally of the latter (capsula externa). The motor fibres from the parietal regions of the cortex are more or less segregated within the internal capsule in accordance with their distribution in the cortex. See Corpus Striatum.

Cauda Equina. The brush-like divergent cluster of fibres springing from near the caudal extremity of the spinal cord.

Caudate Nucleus: see Corpus Striatum.

Central Grey. Relatively undifferentiated grey matter which retains its primitive position near the ventricular surface.

Centrum Semiovale. The oval mass of white matter in the dorsal region of the hemisphere dorsad of the insula.

Cerebellum. The massive organ developed in the roof of the medulla by the concrescence of lateral masses. In man it is corrugated, lobed, and plicated in such a way as to increase enormously its cortical layers. It is intimately connected with equilibration, in which function it is associated with the labyrinthine nerve from the internal ear.

Cerebrum. The paired body developed from the thickened parietes of the embryonic fore-brain, consisting of two hemispheres connected by commissural fibres. The organ par excellence of intelligence, volition, and consciousness.

Chiasma [Opticum]. The crossing (partial decussation) of the optic nerves ventrally of the brain. See NERVOUS SYSTEM (Cranial Nerves).

Cingulum. A band of fibres in the gyrus fornicatus connecting the latter with the olfactory lobes.

Circle of Willis. The polygonally arranged trunk of blood-vessels near the base of the brain (and within it), from which arise the more important arteries of the brain. See BRAIN (Circulation).

Choroid Plexus. An invagination into the brain ventricles of the pia, together with ependymal lining. Cf. Tela: see PLEXUS.

Clark's Column. A cellular column in the dorso-lateral region of the spinal cord connected by fibres with the cerebellum (= columna vesicularis).

Claustrum. A small layer of nerve cells embedded in the external capsule entally of the insula.

Clava. A protuberance in the higher part of the funiculus gracilis containing the nucleus of the funiculus gracilis.

Coelia. Cavities formed by the dilation of the original cavity of the medullary tube; the brain ventricles. See VENTRICULE.

Colliculi. A pair of protuberances into the mesocoele in some lower animals.

Columnae Fornicis. The two columnar masses of fibres from the corpus fornicis, leading thence to the thalamus and corpora mammillaria via tractus corticomammillaris.

Columns: see special article on this topic.

Commissure: see special article on this topic.

Conarium: see Epiphysis.

Conus Terminalis. The conical caudal portion of the spinal cord, extending into the filum terminale.

Convolution: see Gyrus.

Corona Radiata. The sum of the fibres connecting the cortex with lower centres and passing through the capsules of the corpus striatum. The name refers to the appearance presented by the fibres as they radiate from the striatum.

Cornu Ammonis: see Hippocampus.

Cornu Ventriculare. One of the horn-like projections from the lateral ventricles. These are distinguished as the cephalic (or an erior), caudal (or posterior), and ventral (or median or descending) cornua.

[Corpora] Quadrigemina. The four paired eminences upon the roof of the mammalian mesencephalon. The cephalic pair (pregeminum) are in the position of the tectum opticum of lower vertebrates.

[Corpus] Callosum. The dorsal band of commissural fibres connecting the cortex of the two hemispheres, especially (it is claimed) the regions supplying the corona radiata.

[Corpus] Candicans: see Corpus Mammillare.

[Corpus] Dentatum: see Nucleus Dentatus.

Corpus or Nucleus Ectomammillaris. That part of the mammillary lying laterad of the central cell cluster.

Corpus Fornicis. The median (azygos) body formed by the fusion of the crura fornicis. It gives rise to the columnae fornicis ventrally.

[Corpus] Geniculatum. A cell cluster on the lateral aspect of the thalamus connected with the optic fibre system. Two parts (laterale and mediale) are recognized.

Corpus or 'Ganglion' or Nucleus Habenulae. A paired mass of cells in the dorsal part of the thalamus (epithalamus) near the insertion of the epiphysis.

Corpus or Nucleus Interpeduncularis. A cell cluster on the median ventral aspect of the brain base cephalad of the pons within the trigonum interpedunculare; the terminus of Meynert's bundle.

Corpus or Nucleus Mammillaris. A cluster of cells caudad of the infundibulum and within the tuber cinereum. The inferior terminus of the columnae fornicis.

[Corpus] Pineale: see Epiphysis.

[Corpus] Restiforme. One of the caudal peduncles (post-pedunculi) of the cerebellum, forming a protuberance on the dorso-lateral aspect of the medulla.

[Corpus] Striatum. The principal cell aggregate in the base of the hemisphere (see Basal Ganglia). The striata are composed of the caudate and lentiform nuclei, which are separated by the internal capsule (q.v.). The striata are represented in the axial lobes of lower animals.

Corpus Subthalamicum or Corpus Luysi: see Nucleus Subthalamicus.

[Corpus] Trapezoideum. Transverse decussating fibres of the ventral part of the medulla connecting the auditory nuclei of the one side with the lemniscus tract of the other.

Cortex Cerebelli. The superficial layers of grey matter of the cerebellum.

Cortex Cerebri. The external layer of the mantle part of the hemisphere, composed chiefly of cells and their processes, and containing the centres for the voluntary motions and sensations. The organ of consciousness.

Crura Cerebri: see Pedunculi Cerebri.

Crura Fornicis. The fibres of the fornix which connect the hippocampal region with the median corpus fornicis.

Crusta: see Pes Pedunculi.

Culmen. One of the lobules of the vermis cerebelli on the dorsal aspect.

Cuneus. A triangular lobe of the mesal cortex bounded by the fissura parieto-occipitalis and the fissura calcarina.

Decussatio Cerebelli Ventralis. Fibres which cross in the middle line near the ventral aspect of the cerebellum.
The exact components of this decussation remain to be made out.

Decussation. The number of decussations is very great. See special article on this topic.

Deiter's Cells. The constituent elements of the neuroglia.

Deiter's Nucleus. A cell cluster dorsally of the acustic nucleus in the lateral part of the medulla.

Diacoele. The third VENTRICLE (q.v.).

Diencephalon. The thalamus.

Dorsal Plate. One of the regions of the medullary tube. That longitudinal zone forming the dorsal region, usually membranous and in some segments plexiform.

Dorsal Sac: see Paraphysis.

Dorso-median Fasciculus: see Fasciculus Longitudinalis Dorsalis.

Dura [Mater]. The outer membrane of the brain. See BRAIN (Meninges).

Embolus. A mass of grey matter in the cerebellum mesally of the nucleus dentatus.

Encephalon. The brain.

Endyma: see Ependyma.

Epencephalon. The cerebellum.

Ependyma. The (originally ciliated) lining membrane of the coelia.

Epiphysis. The median projection from the dorsal wall of the epithalamus caudad of the supra-commissure. Cf. PARIETAL ORGAN.

Epistriatum. One of the parts of the axial lobe of some low vertebrates.

Epithalamus. The dorsal part of the thalamus containing the nucleus habenulae and other cell groups.

Falx [Cerebri]. A ventrical projection of the dura occupying the longitudinal fissure between the cerebral hemispheres.

Fascia Dentata. The gyrus dentatus.

Fasciculus Communis. A longitudinal fibre system of the medulla of the lower vertebrates, contributing one of the sensory components to the cranial nerves. Probably represented by the fasc solitarius of the mammals. See NERVOUS SYSTEM (Cranial Nerves).

Fasciculus Longitudinalis Dorsalis. A tract of longitudinal fibres collected ventrally of the ventricle, appearing near the region of the nuclei of the third and fourth nerves and extending through the medulla into the spinal cord.

Fasciculus Retrofexus: see Tractus Habenulointerpeduncularis = Meynert's bundle.

Fasciculus Solitarius: see Tractus Solitarius.

Fibrae Arcuatae. Transverse fibres in the medulla. Two portions are recognized -- the externae, lying ectally of the trapezoideum near the ventral surface, and the internae, more deeply situated than the trapezoideum.

Fillet: see Lemniscus.

Filum Olfactorium. One of the fibre bundles of the radix olfactorius (q.v.).

Filum Terminale. The slender caudal prolongation of the spinal cord.

Fimbia. The fibrous zone at the ventral edge of the hippocampus.

Fissura: A furrow of the cortex. Frequently used interchangeably with sulcus. The German Nomenclature Commission propose to limit the term fissure to the more deeply incised sulci of the cerebrum or cerebellum. This distinction is inconstant and leads to confusion. A more satisfactory distinction would be to restrict the term fissure to the cerebrum, and sulcus to the cerebellum, thus embodying in the name a locative index. For the terminology of the fissures see the figures.

Flechsig's Tract. The direct cerebellar tract, or dorso-lateral tract of the cord; an ascending tract lying dorsally of Gower's Tract (q.v.).

Flocculus. One of the lateral projections of the cerebellum. A subordinate portion forms the paraflocculus.

Fluid, Cerebro-spinal. A clear fluid, somewhat resembling lymph, filling the brain ventricles and the subarachnoid space.

Folium Vermis. One of the lobuli of the median lobe of the cerebellum, lying on its dorsal surface. A modern usage applies the term folium generically for gyri of the cerebellum, limiting the application of gyrus to the cerebrum.

Foramen of Magendi. An opening through the dorsal roof of the medulla communicating with the subarachnoid space.

Foramen of Monro: see Portae.

Forceps. That portion of the callosal fibres clothing the caudal cornu of the lateral ventricles.

Fore-brain: see Prosencephalon.

Fornix. A complicated fibre system connecting the hippocampus with other parts of the brain. See Columnae, Corpus and Crura Fornicis, and Fimbria.

Fossa Sylvii. The depression in the lateral aspect of the cortex cerebri in which the insula is situated.

Fovea Limbica. A depressed line separating the olfactory region of the ventral aspect of the hemisphere from the pallium proper.

Fundamental Plate. One of the regions of the medullary tube. A longitudinal zone on either side of the basal plate containing the origins of the motor nerves.

Funiculus Cuneatus. The medullary continuation of the column of Burdach of the spinal cord.

Funiculus Gracilis. The medullary continuation of the column of Goll of the spinal cord. A terete fibre bundle immediately adjacent to the dorso-median line of the medulla.

Ganglion: see special article on this topic. For many current descriptive terms beginning with ganglion see Nucleus.

Gasser's Ganglion. The ganglion on the root of the fifth cranial nerve.

Genu. A purely descriptive term indicating the abrupt flexure of an organ or tract, as the genu of the root fibres of the seventh nerve, or the genu corporis callosi.

Glomerulus Olfactorius. One of the aggregates of fibres formed by the intermingling of the terminal arborizations of the olfactory radices ('nerves') with the dendrites of the mitral cells of the olfactory bulb (pero). See Radix Olfactorius.

Goll's Column. The median dorsal bundle of the cord which is continued into the medulla as the funiculus gracilis.

Gower's Tract. A tract from the spinal cord to the cerebellum; the ascending antero-lateral tract of the cord. By some authors regarded as going to the cerebrum as well as to the cerebellum.

Gyrus. One of the folds or convolutions between the fissures of the cerebral cortex. A gyrus of the cerebellum may conveniently be distinguished as a folium. For the terminology of the gyri see the figures.

Habenula or Habena: see Nucleus Habenulae.

Hemisphere. One of the lateral halves of the cerebrum. Also applied to the lateral lobes of the cerebellum, except the vermis.

Hind-brain. The cerebellum or the cerebellum and pons; sometimes also applied to the cerebellum and medulla or to the medulla alone.

Hippocampus. The cornu ammonis or caudomedian portion of the cerebral cortex, which is curiously conduplicate and partly rolled by a reverse curve into the ventricle. The osmotic cortical region.

Hippocampus minor: see Calcar.

Horn: see Cornu.

Hypophysis. A composite body formed by the union of an outgrowth of the mucous membrane of the fauces with a process from the infundibulum.

Hypothalamus. The ventral portion of the thalamus, including the infundibulum. Contrasted with the epithalamus.

Infundibulum. The ventral projection of the hypothalamus with the corresponding portion of the diacoele. The hypophysis comes into relation with the brain at this point.

Insula [Reili]. A submerged gyrus lying in the Sylvian fossa, and protected (in man) by the opercula.

Isthmus. The constricted part of the brain immediately cephalad of the cerebellum.

Iter. The tube-like remnant of the second embryonic vesicle; the cavity of the mesencephalon, or mesocoele; the aquaeductus Sylvii.

Lamina Terminalis. That part of the median wall of the fore-brain which extends from the preoptic recess to the paraphysis. It is supposed to contain the morphological front, or cephalic extremity, of the nerve tube. See Neuropore.

Laqueus: see Lemniscus.

Lemniscus. The fillet; a bundle of sensory fibres extending from the lateral aspects of the medulla to the mesencephalon. It contains ascending tracts from the several sensory nuclei of the medulla and spinal cord.

Limbic Lobe. The mesal part of the pallium and the ventral structures of the hemispheres, including the hippocampus, septum, gyrus fornicatus, parolfactory region, &c. All of these, except possibly the gyrus fornicatus, seem to be associated with the olfactory function.

Lingula. One of the lobes of the vermis cerebelli.

Lobus. A term applied to poorly defined areas of the cerebrum; a more extensive term than gyrus, as frontal, temporal, occipital lobes.

Lobus Olfactorius. The protuberance of the cerebrum, from which arises the pes olfactorius.

Luys' Body: see Nucleus Subthalamicus.

Lyra [Davidis]. A triangular median area between the callosum and the corpus fornicis.

Mantle: see Pallium.

Mauthner's Fibres. Large fibres in the spinal cord arising in cells connected with the roots of the eighth nerve. They are found in tailed amphibia and fishes, and are probably associated with the power of equilibrium in a fluid medium.

Medulla Oblongata. The prolongation of the spinal cord cephalad which surrounds the metacoele, or chamber formed from the third embryonic vesicle, with the exception of the cerebellum. The pons is sometimes excluded. The medulla and cerebellum together constitute the rhombencephalon. The medulla is primarily the seat of the nerve centres connected with the vital somatic processes.

Medullary tube. The embryonic nervous system at a very early period of its development, immediately after its invagination from the ectoderm, and while still preserving its primitive character of an undifferentiated epithelial tube.

Meninges: see BRAIN.

Mesencephalon. The mid-brain, comprising the corpora quadrigemina tegmentum and pes pedunculi.

Mesocoele: see Iter.

Mesostriatum. One of the parts into which the axial lobe of some vertebrates has been divided.

Metacoele. The fourth VENTRICLE (q.v.)

Metathalamus. That part of the diencephalon forming the transition into the mesencephalon.

Metencephalon. The cerebellum and pons; sometimes also applied to the medulla or to both the cerebellum and the medulla.

Meynert's Bundle: see Tractus Habenulo-interpeduncularis.

Mid-brain. The mesencephalon.

Myelencephalon: see Neuraxis; also applied to the medulla oblongata alone.

Neuraxis. The brain and spinal cord, or cerebrospinal axis.

Neuroblast. An immature nerve cell. Cf. Neurocyte.

Neurocyte. The mature nerve cell. See special article on this topic.

Neuroglia. The supporting tissue of the neuraxis. See special article on this topic.

Neuron: see Neurocyte.

Neuropore. The point where the medullary tube communicates with the outer body surface at the cephalic extremity. A point in the lamina terminalis, supposed to indicate the position of the neuropore, is termed the recessus neuroporicus ( = lobus olfactorius impar, Kupffer).

Nidulus: see NUCLEUS.

Nidus [Avis]. A depression on the ventral surface of the cerebellum. Nidus is also used as a synonym for NUCLEUS (q.v.)

Nodulus. One of the ventro-mesal projections of the cerebellum.

Nucleus: see special article on this topic.

Nucleus Ambiguus. The ventral motor nucleus of the vagus nerve.

Nucleus Amygdalae. A mass of cells at the union of the parietal lobe of the hemisphere with the thalamus; probably associated with the olfactory apparatus.

Nucleus Arcuatus. A cell cluster associated with the fibrae arcuatae of the medulla.

Nucleus Caudatus: see Corpus Striatum.

Nucleus Dentatus. A cluster of nerve cells in the ventro-lateral parts of the vermis cerebelli.

Nucleus Funiculi Cuneati. The terminal nucleus of the column of Burdach.

Nucleus Funiculi Gracilis. The terminal nucleus of the column of Goll.

Nucleus Globosus. A mass of nerve cells in the ventral part of the cerebellum mesally of the embolus.

Nucleus Habenulae. A cell cluster near the roof of the diacoele (third ventricle) near the origin of the epiphysis.

Nucleus Lentiformis or Nucleus Lenticularis: see Corpus Striatum.

Nucleus Niger. A cellular stratum of the mesencephalon between the pes pedunculi and the tegmentum; the substantia nigra Sömmeringi.

Nucleus Pontis. A cell cluster in the pons dorsally of the pyramids and serving as a switch-station between the latter and the cerebellum.

Nucleus Ruber. An important cell cluster in the cephalic part of the tegmentum (mesencephalon).

Nucleus Subthalamicus or Corpus Luysi. A mass of nerve cells in the caudal part of the thalamus near the nucleus ruber and closely associated with the nucleus niger, which replaces it in the pes region.

Nucleus Tegmenti. A cell cluster in the mesoventral part of the cerebellum.

Nucleus Trapezoideus. Cells in the corpus trapezoideum.

Oblongata: see Medulla.

Olfactory Lobe: see Lobus Olfactorius; sometimes also applied to the olfactory bulb.

Olives. A series of corrugated layers of gray matter in the ventral part of the medulla connected with the fillet and cerebellum. They are derived from evaginations of the walls of the primitive medullary tube.

Opercula. Lip-like folds of cortex walling in the Sylvian fossa and covering the insula.

Optic Lobes. The homologues of the cephalic pair of corpora quadrigemina as seen in various lower vertebrates.

Pallium. The brain mantle, or roof of the prosencephalon; it contains the cortex in higher brains, but is membranous in some fishes.

Paracoele. The lateral VENTRICLE (q.v.).

Paraphysis. An evagination of the membranous roof of the fore-brain cephalad of the velum transversum. Some recent writers denominate this the 'preparaphysis' to distinguish it from a similar evagination of the tela between the velum transversum and the supra-commissure, the 'post-paraphysis.' The latter is more properly termed 'dorsal sac'; it is the Zirbelpolster of the Germans. The morphological significance of these structures is intimately bound up with that of the epiphysis and PARIETAL ORGAN (q.v.).

Pedunculi Cerebelli. Three pairs of fibre bundles connecting the cerebellum with the other parts of the brain. The cephalic pair (prepeduncles, or brachia conjunctiva cerebelli anterior) can be followed cephalad to the tegmentum; the medi-pedunculi (brachia pontis, or processi cerebelli ad pontem) decussate ventrally, forming the pons; while the post-pedunculi (brachia conjunctiva cerebelli posterior) form the corpora restiformia.

Pedunculi Cerebri. The compact fibre bundles connecting the hemispheres with the lower centres and appearing as convergent prominences on the ventral surfaces of the mesencephalon. See Pes Pedunculi.

Perforata or Perforated Space. An area in which numerous blood-vessels enter the brain. The two most important are the preperforata in the area olfactoria and the post-perforata in the trigonum interpedunculare.

Pero Olfactorius. The ectal portion of the olfactory bulb, including the glomerules and mitral cell zone; the formatio bulbaris of some authors.

Pes Olfactorius. The ental part of the olfactory bulb.

Pes Pedunculi. The pedunculi cerebri in their course through the mesencephalon where they appear on the ventral aspect leaving between them the trigonum interpedunculi.

Pia [Mater]. The inner membrane of the brain. See BRAIN (Meninges).

Pillars of the Fornix: see Columnae Fornicis.

Pineal: see Epiphysis.

Pituitary: see Hypophysis.

Plexus. An anastomosis of nerves outside the brain. See special article on this topic.

Plexus Choroideus: see Choroid Plexus.

Pons [Varolii]. The bundle of transverse fibres crossing the ventral aspect of the medulla near its cephalic end and composed largely of the middle peduncle fibres of the cerebellum.

Portae. The two openings from the aula into the lateral ventricles.

Post-geminum. The caudal pair of corpora quadrigemina.

Pregeminum. The cephalic pair of corpora quadrigemina.

Prosencephalon. The secondary fore-brain, including the striata.

Psalterium. The commissura hippocampi.

Pseudocoele. A remnant of the original cleft separating the two cerebral hemispheres included between the two laminae of the septum, the fifth ventricle.

Pulvinar. A tuberosity of the dorso-lateral aspect of the thalamus belonging to the optic system and associated with the geniculatum.

Purkinje's Cells. Large, globose cells characteristic of the cerebellar cortex.

Putamen. A distinct part of the nucleus lentiformis.

Pyramids: see Tractus Cortico-spinalis.

Radiatio Strio-thalamica. Fibres connecting the striatum (caudate nucleus) and the thalamic and mesencephalic nuclei.

Radiatio Thalamo-occipitalis ( = Optic Radation or Gradiolet's Bundle). A tract from the primary optic centres (geniculata, &c.) to the cortex about the cuneus.

Radix: see special article on this topic.

Radix Olfactorius. The so-called olfactory nerve, which is now supposed by some to consist of the root portion only, the ganglion being scattered in the mucous membrane of the nose; formerly, however, often applied to the tractus olfactorius.

Raphe. A continuous decussation in the median plane of the medulla.

Recessus Infundibularis. The cavity of the infundibulum; a part of the diacoele.

Recessus Mammillaris. The projection of the diacoele into the mammillary region of the thalamus.

Recessi Post- and Pre-optici. The projections of the diacoele caudad and cephalad of the chiasm.

Rhinencephalon. The olfactory bulb and tractus; sometimes also extended to include the lobus olfactorius.

Rhombencephalon. The medulla, pons, and cerebellum.

Saccus Vasculosus. A vascular plexiform protrusion of the ventro-mesal wall of the thalamus, in some animals caudad of the hypophysis and between that body and the mammillaria.

Septum [Pellucidum]. The thin median partition separating the hemispheres cephalad of the aula. It contains the pseudocoele.

Spongioblast. One of the epithelium cells which constitute the embryonic nerve tube and give rise to the ependyma and neuroglia of the adult.

Stem. All of the brain except the pallium and the cerebellum.

Stratum Lucidum. The middle zone of the gyrus hippocampi.

Stratum Zonale [Thalami]. The superficial fibre layer on the dorsal aspect of the thalamus.

Striae Acusticae. Transverse fibres connecting the accessory acustic nucleus of one side with the lemniscus of the other.

Striae Lancisii. Fibres which connect the hippocampus with the lobus olfactorius by way of the dorsal surface of the callosum.

Striae Medullares: see Striae Asuticae.

Subarachnoid Space. The space between the arachnoid and the pia and in communication with the brain ventricles through the foramen of Magendi.

Substantia Gelatinosa [Rolandi]. An area of the spinal cord near the apex of the dorsal cornu and in the path of the root fibres.

Substantia Nigra [Sömmeringi]: see Nucleus Niger.

Substantia Perforata: see Perforata.

Substantia Recticularis. The region beginning in the tegmentum at the level of the nucleus ruber and extending caudad into the medulla ventrally of the ventricle.

Sulcus: see Fissura.

Sylvian Fossa: see Fossa Sylvii.

Taenia Thalami. A slender band of fibres connecting the hippocampus with the habenulae.

Tapetum. Fibres clothing the caudal and ventral cornua of the lateral ventricles.

Tectum Opticum. The roof of the mesencephalic ventricle in lower vertebrates. Its place is largely occupied by the cephalic pair of the corpora quadrigemina in mammals.

Tegmentum. The infra-ventricular region of the mesencephalon dorsad of the pes pedunculi.

Tela [Choroidea]. A membranous part of the roof of the brain, especially the roof of the thalamus and parts adjacent, from which the choroid plexus is developed. See PLEXUS.

Telencephalon: see Prosencephalon.

Tentorium [Cerebelli]. A transverse projection of the dura lying between the cerebrum and the cerebellum.

Terma: see Lamina Terminalis.

Thalamus. The walls of the diencephalon. Divided into hypo-, meta-, and epi-thalamus. The thalamus contains many important primary sensory centres.

Tonsilla. Lobes on the ventral aspect of the cerebellum laterally of the uvula.

Torus Longitudinalis. A paired ridge on either side of the medial line of the tectum, projecting into the mesencephalic ventricle, in fishes.

Torus Semicircularis. Lateral projections from the walls of the mesencephalic ventricle in fishes.

Tracts. Bundles of fibres with a common origin and destination. The list below is incomplete, and where the name contains the termini no description is added.

Tractus Acustico-spinalis. Mauthner's fibres.

Tractus Acustico-tectalis. Part of lemniscus.

Tractus Bulbo-corticalis. Olfactory radiation.

Tractus Bulbo-epistriaticus. Olfactory radiation.

Tractus Cerebello-olivaris.

Tractus Cerebello-spinalis. Flechsig's tract.

Tractus Cerebello-tegmentalis.

Tractus Cortico-epistriaticus.

Tractus Cortico-habenularis. Taenia thalami in part.

Tractus Cortico-habenulo-peduncularis.

Tractus Cortico-mammillaris: see Columnae Fornicis.

Tractus Cortico-spinalis. The great motor paths from the cerebrum to the spinal cord and appearing as pyramidal elevations on the ventral surface of the medulla; the pyramids.

Tractus Cortico-tegmentalis.

Tractus Cortico-thalamicus.

Tractus Habenulo-interpeduncularis. Meynert's bundle.

Tractus Intermedio-lateralis. The lateral cornu of the cord.

Tractus Mammillo-peduncularis.

Tractus Nucleo-cerebellaris.

Tractus Occipito-mesencephalicus. Optic radiations.

Tractus Olfacto-habenularis. Taenia thalami in part.

Tractus Olfactorius.

Tractus Olfactorius Lateralis.

Tractus Olfactorius Septi.

Tractus Opticus. The fibres of the optic nerve as they spread out over the lateral aspect of the thalamus.

Tractus Peduncularis Transversus.

Tractus Septo-mesencephalicus.

Tractus Solitarius. The common descending root of the vagus and glossopharyngeal nerves. Cf. Fasciculus Communis.

Tractus Strio-lobaris.

Tractus Tecto-bulbaris.

Tractus Tecto- et Thalamo-spinalis. Part of lemniscus.

Tractus Tecto-nuclearis.

Tractus Tecto-spinalis. Part of lemniscus.

Tractus Tecto-thalamicus.

Tractus Tegmento-cerebellaris.

Tractus Thalamo-bulbaris et -spinalis. Part of lemniscus.

Tractus Thalamo-mammillaris. Vicq d'Azyr's bundle.

Tractus Vago-cerebellaris. Secondary vagus bundle.

Trigonum Interpedunculare. The triangular space between the pes pedunculi.

Tuber Cinereum. An excrescence on the ventral aspect of the thalamus caudad of the infundibulum.

Tuber Olfactorium: see Bulbus Olfactorius.

Uncus. A protuberant part of the hippocampus; the gyrus uncinatus.

Uvula. One of the lobules on the ventral aspect of the median lobe of the cerebellum.

Valvula. A valve-like projection from the cephalic edge of the cerebellum at its union with the velum medullare cephale.

Velum. A thin membranous state of the dorsal part of the walls of the medullary tube: cf. Tela. Especially Velum Medullare Cephale, the thin roof of the metacoele cephalad of the cerebellum; Velum Medullare Caudale, the thin roof of the metacoele caudad of the cerebellum; and Velum Transversum, a transverse fold of the tela extending into the diacoele. The latter is commonly assumed as the arbitrary boundary between the prosencephalon and the diencephalon.

Ventricles: see special article on this topic.

Ventriculus Septi [Pellucidi]: see Pseudocoele.

Vermis Cerebelli. The large median lobe of the cerebellum.

Vicq d'Azyr's Bundle: see Tractus Thalamomammilaria.

Volvula. A massive projection of the cerebellum thrust within the mesencephalic ventricle of fishes. (Also called Valvula.)

Worm: see Vermis.

Zona Granulosa [Cort. Cerebelli]. The deeper layer of the cerebellum below the cells of Purkinje.

Zona Molecularis [Cort. Cerebelli]. The peripheral layer of the cortex of the cerebellum in which the dendrites of the cells of Purkinje ramify.

Zona Spongiosa. That part of the dorsal cornu of the cord between the zona terminalis and the substantia gelatinosa.

Zona Terminalis. That region of the dorsal cornua of the spinal cord where the sensory nerve fibres enter. (H.H.)

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