32 PHYSIOLOGY [NERVOUS the fifth ventricle. By a thickening of the floor of the middle cerebral vesicle (meaencephalon) two large bundles of longitudinal fibres, the crura cerebri, are formed, whilst its roof is modified into the optic lobes, corpora bigcmina or corpora quadrigemina. The cavity, reduced to a mere tube, is the itcr a tcrtio ad quart um vcn- triculum, or the aqueduct of Sylvius. The third cerebral vesicle, mylcnccptuilon, undergoes less modification than the others. The upper wall is exceedingly thin before the cerebellum so as to form a lamina, the valve of Vieusscius, whilst the part behind is covered only by membrane, and opens into the posterior subarachnoid space. The cerebellum makes its appearance as a thin medullary lamina, forming an arch behind the corpora quadrigemina across the wide primitive medullary tube. The portion forming cerebellum, pons Varolii, and the anterior part of the fourth ventricle is termed the cpenccphalon, whilst the remaining portion, forming the medulla oblongata and fourth ventricle, is the metcnccphalon. These facts are briefly summarized as follows (Quain, vol. ii. p. 828). {Cerebral hemispheres, corpora striata, corpus callosum, fornix, lateral ventricles, olfactory bulb (rhinencc.phalon). vesicle. "*. b. Thalamenceph- ( Optic thalami, pineal gland, pitui- alon Inter- ! tary body, third ventricle, optic V. brain. ( nerve (primarily). 2. Middle cerebral f c. Mesencephalon ( ^"a.^lncf of^lvIT" onto -Mid-brain. { *ilf$SSlf* id. Epencephalon j Cerebellum, pons Varolii, anterior Hind-brain. ( part of the fourth ventricle. e. Metencephalon j Medulla oblongata, fourth ven- After-brain. tricle, auditory nerve. The general architecture of the brain considered in this way will be understood by the diagram in fig. 15, whilst details as to the
Fio. 15. Diagrammatic longitudinal and vertical section of a vertebrate brain. The lamina terminalis is represented by the strong black line between FM and 3. Ifb, mid-brain, what lies in front of this being the fore-brain, and what lies behind the hind-brain ; Olf, the olfactory lobes ; Hmp, the hemispheres ; ThE, the thalamencephalon ; Pn, the pineal gland ; Py, the pituitary body ; FM, the foramen of Monro ; CS, the corpus striatum ; Tk, the optic thalamu.s ; CQ, the corpora quadrigemina ; t C, the crura cerebri ; Cb, the cerebellum ; PV, the pons Varolii ; MO, the medulla oblongata ; I, olfactorii ; II, optici ; III, point, of exit from the brain of the motores oculorum ; IV, of the pathetici ; V, of the abducentes ; VI-XII, origins of the other cerebral nerves ; 1, olfactory ventricle ; 2, lateral ventricle ; , third ventricle ; 4, fourth ven tricle. (Huxley.) exact anatomy of the human brain will be found under ANATOMY (vol. i. p. 869 sq. ). The complex structure of the brain in the higher animals arises to a large extent from the great development of the cerebral hemi spheres. At a very early period these grow forward and project more and more beyond the region of the first primary vesicle, which, as has been noticed, never ad- vances farther forward than the pituitary fossa (lamina terminalis) ; in expanding upwards they take the place previously occupied by the mid -brain, and fill the most prominent part of the head ; and by a downward and lateral enlargement L Fio. 16. Surface of foetal brain at six months (from R. Wagner). This figure is intended to show the commencement of formation of the principal fissures and convolutions. A, from above ; S, from left side. F, frontal lobe ; 1 , parietal ; 0, occipital ; T, temporal ; a, a, a, slight appearance of several frontal convolutions ; s, bylvian fissure ; s , its anterior division ; within C, central lobe or convolutions of island of Reil ; r, fissure of Rolando; p, l>arieto-occipital fissure. (Quain.) they form the temporal lobes. Thus frontal, parietal, and tem poral lobes come to be distinguishable, and somewhat later, by a farther increase posteriorly, the hindmost lobes constitute the occipital lobes, and the cerebrum at last covers completely all the lower parts of the brain. The hemispheres, therefore, which are small iii the early embryo of all animals, and in adult fishes ]>erma- nently, attain so large a size in man and in the higher animals as to conceal all the other parts. Whilst this general development is going on the layer of grey matter on the surface of the hemispheres increases to such an extent as to throw the surface into folds or con volutions. The upper surface of the hemispheres is at first smooth (see fig. 16). The first appearance of division into lobes is that of a blunt notch between the frontal and temporal parts below, in what afterwards becomes the Sylvian fissure. In the fourth and fifth months there appear the vertical fissure, separating the parietal and occipital lobes, and the transverse fissure, called the fissure of Rolando, which divides the frontal and parietal lobes superiorly, and which is peculiarly characteristic of the cerebral type of man and of the apes (Allen Thomson). Then the convolutions appear from the formation of secondary grooves or sulci, for even at birth they are not fully perfected ; and by the deepening of the grooves and the formation of subordinate ones the process goes on during the first years of infancy. For the convolutions see vol. i. p. 873 ; also PHRENOLOGY, vol. xviii. p. 847. The evolution of the brain throughout the animal kingdom shows Develo] a graduated series of increasing complication proceeding out of the nient o: same fundamental type ; so that the forms of brain found perma- brain ii nently in fishes, amphibians, reptiles, birds, and in the lower mam- animal mals are repetitions of those shown in the stages of the embryonic series, development of the brain of one of the higher animals. In the whole class of fishes the brain retains throughout life more or less of the elementary form, that is, it consists of a series of enlargements, single or in pairs (see fig. 17, C). The simplest of all forms is in the lancelet (Branchio- stoma), in which there is no distinction between brain and cord, there being no anterior enlargement to form an encephalon. In the Cyclostomata, as the lampreys, the form is nearer that of the embryo when the five fundamental ,.,,,. , Fio. 17. Typical forms of brains of lower parts Of the _ brain can DC vertebrates. A. Brain of tortoise (Testudu). 1, olfactory ; 2, cerebral lobes ; 3, corpora striata ; 4, optic lobes ; 5, medulla. Part of the surface of the cerebral lobes has been removed t,o show the cavities in the interior, termed "the ventricles." Immediately be hind 4, the optic lobes, is the imperfectly- developed cerebellum. B. Brain of common frog (liana), a, olfactory ; b, cerebral lobes covering corpora striata ; c, corpora quadri gemina, or optic lobes ; d, cerebellum (rudi mentary) ; s, back of medulla, showing fossa. C. Brain of gurnard (Triyla). 1, olfactory; 2, cerebral lobes ; 3, optic lobes ; 4, cerebellum. distinguished. At this stage the cerebrum and cerebellum are extremely small, whilst the ganglia chiefly developed are those connected with the organs of sense, more especially those of vision. In the sharks and skates (SelacJiii, or cartilaginous fishes) the cerebral portion is consi derably larger. In osseous fishes ( Tclcostci] the thalamencephalon is so fused with the mesencephalon as to make the homologv of the parts difficult to trace, but both cerebellum and cerebrum are still small relatively to the rest of the brain. The most important part of the brain of a fish is the part behind the mesencephalon, as from it all the cerebral nerves originate. Thus not only are the optic lobes relatively important as being the centres of vision, but the medulla oblongata is usually very large. In many sharks it forms the largest part of the brain (Gegenbauer). The spinal lobes of the electric fishes are differentiations of this portion of the encephalon. In the Amphibia the hemispheres are larger, and are divided into two parts (see fig. 17, B). In the Urodcla (siren, proteus, triton, newt) the mesencephalon remains small, and consists of one lobe, but in the Anura (frogs, toads, &c. ) there is an advance in this part, it being divided into two. In reptiles there is still an advance in the size of the thalamencephalon and mesencephalon, and the pros- encephalon is so large as to pass backwards and overlap the thalam encephalon. The cerebellum (metencephalon) is still small, especially so in Ophidii (serpents) and Saurii (lizards), but in the Chclonii (tortoises, &;c. ) and in Crocodilini (crocodiles, alligators) it is larger. In the crocodile there is a transverse grooving of the cerebellum, giving rise to foliation or laminar division, which is carried much farther in birds and mammals, indicating a greater power of co-ordination or regulation of movement. In birds (fig. 18) the vesicles of the mid-brain are partially hidden by development of the cerebral hemispheres. These are connected by a fine anterior commissure, and they contain a large amount of ganglionic matter bulging into the primitive cavity or ventricles, which are of very small size. The middle portion of the cerebellum shows a distinctly laminated structure and a differentiation into white and grey matter. But there is no pons Varolii, nor corpus
callosum, nor fornix, nor hippocampus. In the floor of the lateral
