Each hemisphere is a bag of neurine folded or laid upon its expanding stem, the hollow of the bag being the ventricle. This, in the embryo, is capacious and simple, the wall being very thin. It becomes thickened in different degrees at different places, most so at the upper and outer sides. The wall, thus thickened, protrudes at certain parts into the cavity, dividing and shaping it into parts or recesses which Anthropotomy calls horns,' from their curvature in Man. In lower Mammals the primitive cavity commonly retains more of the general shape of the hemisphere, and in most Quadrumana, the lower more especially, the part accompanying the broad supracerebellar expansion of the hemisphere is of corresponding capacity. The Orang, among Apes, still shows the primitive character of this part of the ventricle: in the Chimpanzee and Gorilla the growing walls reduce and begin to shape it as a horn,' showing also a beginning of a protuberance within it. In Archencephala the moulding of the 'posterior horn' is completed by the predominance of the internally protruding wall ('partie enroulée,' Leuret), to which, now, the hippocampus minor,' or 'pes hippocampi minor,' rightly applies. The fibres of the stem, augmented in number at each accumulation of grey reuniting matter, diverge into and form the main part of the wall in greatest proportion in the Lyencephala.

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The stem or crus 'is formed by the prepyramidal tracts, fig. 66, p, the olivary tracts f, the teretial and postpyramidal tracts, fig. 49, Y, and so much of the cerebellar tracts, fig. 66, t, as may not have been expended in the formation of the nates,' b, testes,' n, 'geniculate bodies,' y, and their common basis. Thus the crus or stem of the hemisphere includes tracts of the myelon, connected respectively with the sensory and motory roots of the nerves. The part of the crus prosencephali,' below or in front of the 'locus niger,' consists of white fibres in a coarsely fasciculate ' arrangement, fig. 123, d: the part above, derived from the teretial, postpyramidal, and cerebellar tracts, is softer, with mixed grey matter, and forms the tegmentum,' ib. c. The fasciculate fibres, after passing through and being reinforced by the grey matter of the striated body, diverge in curves, fig. 66, c, fig..122, s,

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The judicious and painstaking anatomist GRATIOLET seems to have foreseen some late misconceptions of the nature of the hind part of the primitive ventricular cavity in the Quadrumanous brain, in the following note: Toutefois, il ne peut être considéré comme un signe d'élévation, car il est beaucoup plus grand en égard à la partie enroulée du ventricule dans les singes, où son développement est énorme, que dans l'homme, où la partie enroulée l'emporte évidemment sur lui. Cette remarque,' he justly adds, est d'une haute importance.' XL". vol. ii. p. 75.

of which many bend downward and outward, suggesting the term 'fibrous' or 'radiated' cone; in Man they are traceable chiefly in the sylvian, postsylvian, entosylvian, supersylvian, medilateral, medial, and marginal folds, and into the major part of those of the anterior lobe, fig. 122, a. The tegmental or posterior fibres are, in Man, more directly connected with the transversely arched

Dissection of cerebrum and cerebellum, from the outer side. XXXIII".

fibres of the great commissure: others, diverging to the posterior lobes, e, b, become connected or continuous with the longitudinal commissural system of the fornix. Figure 123 is a dissection of the inner surface of the hemisphere. c is the section of the corpus callosum, the fibres of which diverge upon the roof of the ventricle, intersecting the radiating fibres, fig. 122, s, and passing into all the folds, which are thus brought into communication with those of the opposite hemisphere. The fibres of the callosal' fold, fig. 123, o, o, are chiefly longitudinal, are continued behind, into those of the hippocampus, and in front into those extending from the fornix upon the falcial surface of the anterior lobe: externally

spread over the mesial surface of the anterior lobes, and are homologous with those marked o' in fig. 75, and in figs. 4 and 6, pl. vi. LXX.

The corpus callosum being removed, and the commissural fibres of the hippocampi being left behind, the view of the Beaver's brain now corresponds with that obtained in the dissection of the brain of the Wombat, fig. 75. The artery of the plexus choroides, ib. p, in both the Beaver and Wombat, enters the lateral ventricle, where the hippocampus commences at the base of the hemisphere, and the plexus is continued along the under surface of the tænia hippocampi, and passes beneath the fornix, through the usual foramen, to communicate with its fellow in the third ventricle immediately behind the anterior crura of the fornix, which are sent down in the Beaver, as in the Wombat, from the centre of the inferior surface of the hippocampal commissure.

If we expose the lateral ventricle by removing its outer parietes in a marsupial and placental quadruped, the hippocampus major, the tænia hippocampi, the plexus choroides, and the foramen Monroianum are brought into view. If a style be thrust transversely through the internal wall of the ventricle, immediately above the hippocampal commissure, in the placental quadruped, it enters the opposite ventricle and perforates the septum lucidum, passing below the corpus callosum. If the same be done in the Marsupial brain, the style passes into the opposite ventricle, but is immediately brought into view from above by divaricating the hemispheres.

The commissure, answering to the lyra,' represents the beginning of the corpus callosum; but this determination does not invalidate the fact that the great commissure which unites the supraventricular masses in the Hedgehog, Beaver, Bat, and all other placentally developed mammals is a definite superaddition for more effectually associating the hemispheres in whatever motion or change they may undergo in the actions of the brain.

All Lissencephala show a large proportional size of the hippocampi, fig. 79, f and e, a small corpus striatum,' d, and large bigeminal bodies,' k, which bear the same proportion to each other as in Marsupialia. The smaller the brain, the larger is the share which the mesencephalon takes in its formation, as in the Shrews and Moles (fig. 67).

The rhinencephalon, fig. 79, g, fig. 81, e, is connected by a broad and complex crus' with the under part of the hemispheres,

having the outer root,' fig. 82, x, which is continued from the

The chief brain-characters of the Lissencephala are, the nonextension of the cerebrum over the cerebellum, the paucity and

Base of the brain, Agouti. XXXII". Base of the brain, Porcupine (Hystrix cristata). XXXII".

simplicity of folds on the exposed surface of the hemispheres in a few, their absence in most; the connection of the two hemispheres by a corpus callosum,' as well as by the lyra' and anterior commissure,' the absence of the septum lucidum,' and the proportionally large hippocampi and bigeminal bodies.

C. Gyrencephala.- In this subclass the prosencephalon is relatively larger, extending backward more or less

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over the cerebellum with a concomitantly developed corpus callosum,' the connection of which with the fornix' is now maintained, not only by the lyra,' but by the attenuated vertically extended subjacent parts of the medial walls of the lateral ventricles called septum lucidum,' their interspace being the fifth ventricle' of Anthropotomy, fig. 118, n.

In some of the smallest species of Gyrencephala the exposed surface of the cerebral hemispheres may be smooth, or with few and simple fissures (fig. 96, Hyrax; fig. 101, Tragulus; and vol. ii. fig. 147). This state does not, however, relate to reduction of hemispheres,

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but may coexist with their extension over the whole cerebellum,' as in some small Quadrumana, fig. 109, Midas and Callithrix; but the increase of superficial grey matter by fissures and folds is now the rule.

Three leading patterns of convoluted surface, which, from the prevalent direction of fissuring, may be termed the oblique,' longitudinal,' and 'transverse,' are presented by the Gyrencephala, and are exemplified, respectively, in the ungulate, unguiculate, and quadrumanous divisions of the subclass. Notwithstanding, in these general variations homologous

LXX. pl. v, fig 2 (1836).