the cerebrum. In the Leporida, fig. 79, a, it is lozenge-shaped, with the anterior borders longer, and converging to a narrower (though obtuse) apex, than the posterior ones. In the Pacas the cerebrum is broader, with both ends more obtuse and larger, and the hinder third is broader. In Castor, fig. 78, it pre

77

sents a full ovate figure. In
Hystrix, fig. 77, it is subquadrate,
through increasing breadth of the
fore part.
On the medial surface
of the hemisphere the hippocampal
fissure' is confined to the hinder
half; the callosal fissure' is super-
added, commencing at the sple-
nium' or posterior genu of the
great commissure, and running
along its upper surface to the an-
terior genu; it is shallow, but now
defines the true labium cerebri.'
On the under surface the ectorhinal
fissure, figs. 83 and 84, 2, has the
same extent as in the Wombat; it

Upper surface of the brain of the Porcupine. diverges, as it recedes, further from its fellow, in fig. 83, through the greater breadth of the basirhinal protuberances, h. A few short fissures rise from its anterior half a little way upon the hemisphere in the Cavies, as in the Wombat. In the Porcupine the sylvian fissure, figs. 77, 84, 5, is well marked, though short, and there is a feeble indication of the coronal fissure,' 12.

In the smaller and especially the insectivorous Bruta the brain presents the lissencephalous type, having smooth, low, triangular hemispheres, leaving the mesencephalon as well as epencephalon in view posteriorly. Dasypus has the fore part less contracted than Myrmecophaga, at least than M. didactyla. In Bradypus the anterior expansion gives an ovate form to the hemispheres; and now, besides the hippocampal, callosal, ectorhinal, and sylvian fissures, the upper surface shows the medilateral, suprasylvian, and frontal ones. The medilateral bends outward anteriorly, defining the anterior lobe' impressed by the short angular or triradiate frontal' fissure. The above fissures mark out a medial, lateral, sylvian, postfrontal, and prefrontal convolutions. On the inner surface a supercallosal

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fissure now defines a convolution of the corpus callosum," the lower margin of which, resting on that body, is the labium cerebri' of anthropotomy: in Sloths, as in Shrews and Rodents, it is anterior to and distinct from the hippocampal' fissure and fold.

In all Lissencephala the hemispheres present the following structure, the rise in which, as compared with that in Lyencephala, is independent of the smallness and smoothness of those divisions of the brain.

Taking that of the Beaver (Castor fiber), e. g., and comparing its prosencephalon with the sub-convolute one in Phascolomys or Macropus, we find, on divaricating the hemispheres, that the corpus callosum' is brought into view, and on removing the cerebral substance to a level with this body, as in fig. 78, its fibres are observed to diverge into the substance of each hemisphere, some bending upward, but a greater proportion arching downward and commingling with those that diverge from the cerebral nuclei. The portions of the brain which are removed in thus tracing the extent of the corpus callosum bring into view the corpora bigemina, B, and the pineal gland, u; but the optic thalami are concealed by the great com

78

missure above described. If the posterior margin of the corpus callosum be raised, its inferior surface is found to be closely connected or continuous with the transverse commissural band of fibres, arching over the anterior part of the optic thalami, and passing outward and backward along the floor of the lateral ventricles into the substance of the hippocampi, which are almost as large as in the Wombat. The anterior part of the corpus callosum is bent downward, and it is attached along the middle line of its inferior surface by a uniting medium of medullary substance, the beginning of the septum lucidum,' to the hippocampal commissure or fornix; the tæniæ hippocampi send forward, as in the Wombat, a delicate layer of medullary fibres which

''Ourlet' and 'internal convolution' of Foville, xxv".

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,

6

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

79

h

basirhinal tract,' ib. h, the inner root,' y, and the intermediate perforate body or tract,' ib. r: the lateral ventricle is continued into the rhinencephalon (fig. 79, h).

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".

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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.

83

6

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

Brain of Cat, Felis domestica.

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,

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.