73

has the anterior apex of the hemisphere marked off by a deeper transverse fissure, extending to the inner surface. In the Herbivorous Marsupials the fissures are more definite, deeper, and rather more numerous in the larger (Macropus major, fig. 74) than in the smaller species (Hypsiprymnus). All Marsupials have the hippocampal fissure, fig. 46, 4, fig. 73, i, coextensive

Didelphys Virginiana.

74

with the antero-posterior range of the prosencephalic cavity, and arching over all the commissural apparatus of the hemispheres. The concomitant extent of the convolution (hippocampus major) is shown in LXX'. pl. vii. figs. 3 (Didelphys) and 4 (Macropus), in the exposure of the ventricle from the outer side. In Didelphys, fig. 73, the surface of the hemisphere above the fissure is feebly impressed by blood-vessels; in Thylacinus there is a short fissure above the back part of the hippocampal one; in Phascolomys and Macropus there is also an anterior one which bends or bifurcates at its fore part. These fissures mark the level of the roof of the lateral ventricle; the surface below forming the thin mesial wall of the cavity, fig. 75, q, which in the higher Placentals is defined, as the 'septum lucidum,' by a corpus callosum from the part above, On the upper surface of the hemisphere, in Macropus major, a longitudinal part of the fissure, fig. 74, 8, marks off a medial convolution, 7, at the anterior half, and occasionally it is prolonged backward by the fissure, 10, as in the left hemisphere of fig. 74. But there is continued from 8, in both hemispheres, a fissure extending outward, which bounds behind the

part of the hemisphere impressed by the 'sylvian fissure,' 5. The

LXX'. pl. vi. figs. 4 and 6, q, q.

sylvian convolution, e, e', folded on 5, is divided behind by the fissure, 9, from the post-sylvian fold, f. The contracted anterior part of the hemisphere is marked off by a feeble coronal fissure, 12, and is partially divided into a superfrontal fold, n*, and a subfrontal fold, n'. In the broad hind part of the hemisphere are the medial, 1, and lateral, m, tracts.

On separating the hemispheres of the brain of the Wombat, not only the bigeminal bodies, B, fig. 75, and pineal gland, ib. u, but the thalami, ib. t, t, are brought into view, and instead of a broad corpus callosum, we perceive, situated deeply, a small commissural medullary band, ib. m, passing in an arched form over the anterior part of the thalami, and extending beneath the labia hippocampi,' the supraventricular part of the hemi

spheres, q, being thus, as in the bird and monotreme, disconnected with each other. On gently raising the labia' from above the commissure and pressing them outward with the handle of a scalpel, the instrument passes into the fissure upon which the hippocampus, ib. n, is folded. The mesial wall of the hemisphere is continued from the upper labium of the hippocampus, and is composed of a thin lamina of medullary substance analogous to a detached layer of the septum lucidum. In the Kangaroo the mesial parietes of the lateral ventricles are

thicker. In both Marsupials they receive the thin layer of fibres, fig. 75, o, passing from the commissure, m, over the upper lip of the hippocampal fold, to radiate vertically upon the anterior half of the inner wall of the ventricle. These fibres are figured in LXX'. pl. vi. fig. 4, o' (Wombat), and fig. 6, o' (Kangaroo), and are described as the anterior fibres of the tænia hippocampi continued into the anterior lobes of the hemispheres."2

These are not to be confounded with the labia cerebri' of anthropotomy.

2 The author of XLIII". figures, in pl. xxxvi. fig. 4, a more extensive series of transverse fibres which he describes, p. 644, as being lost beneath the labia cerebri,' as the margin of the callosal fold' is called by some authors. The surface of the diva

The crura cerebri, which, in the Opossum, e, fig. 53, are left exposed below, like the optic lobes above, by reason of the small proportional size of the cerebrum, are more completely concealed in the brain of the Kangaroo and Wombat. The natiform protuberances form a great proportion of the under part of the cerebral hemispheres in all the Marsupials; the ectorhinal fissure which indents their base in the Wombat and Kangaroo, runs along the side of the hemisphere to the outer side of the olfactory lobe in the Opossum, indicating the large relative size of the basirhinal fold or tract. Behind the commissure of the optic nerves is seen a broad and short infundibulum supporting the pituitary body, g, fig. 53, and posterior to this is the single corpus albicans. The optic lobes, fig. 73, b, are solid; a pair of similar but smaller ones rise behind, and form with them a 'bigeminal' mass: the anterior divisions ornates,' B, fig. 75, have a greater longitudinal diameter than the posterior ones or testes,' which have a greater transverse developement. The difference in the relative developement of the nates and testes between the herbivorous and carnivorous Marsupials is less than in the corresponding Placental quadrupeds.

The posterior transverse fibres of the hippocampal commissure are continued, fig. 75, m, outward and backward beneath the more longitudinal fibres, which overlap them as they pass forward to the anterior cerebral lobes and pass into the substance of the hippocampi, n. Thus the commissure, which is brought into view on divaricating the cerebral hemispheres in the Wombat, is seen to be partly the bond of union of the two hippocampi majores in the transverse direction, like the lyra' of anthropotomy, and partly of the hippocampus and the fore and inner parts of the hemisphere in the longitudinal and vertical directions. It mainly fulfils the function of the fornix, not only as being the hippocampal commissure and continued backward and downward as 'posterior pillars; but by sending down from the inferior surface two small nerve-like processes, which extend vertically, behind the anterior commissure, to the corpus albicans,

ricated hemispheres is left entire, and whether the fibres diverge into the substance of the roof of the ventricles is not shown. In LXX'. pl. vi. figs. 4 and 6, the requisite dissection is made and figured, and the transverse fibres are shown to be lost beneath the labia hippocampi '—that is, to be continued into the hippocampi, not into the supraventricular substance. Other dissectors of the brain of a Macropus Benettii and of Phascolomys might compare the appearances with those figured in the Philosophical Transactions,' 1837, pl. vi. figs. 4 and 6, and in the Philosophical Transactions,' 1865, pl. xxxvi, fig. 4.

at the base of the brain. The superior view of the connections of the hippocampal commissure of the Wombat is given at m, n, o, fig. 75.1

The artery of the plexus choroides, entering with the fold of pia mater at the lowest part of the hippocampus, is richly spread upon the small production of that fold, ib. p, beneath the margin of the 'tænia' near the passage by which it is continued into the fold and plexus of the opposite ventricle. This intercommunication between the two prosencephalic cavities exists in all Mammals, and is defined in anthropotomy as the foramen Monroianum.' The pineal appendage, ib. u, is small compared with its 'crura,' which, as in all other Mammals, are continued backward from the fore and inner parts of the thalami, t.

A well-marked ectorhinal fissure extends from the natiform protuberance, defining externally its basirhinal tract'2 and the forward continuation of the crus rhinencephali.' A longitudinal white streak divides the outer and inner portions of that crus. The prosencephalic cavity is continued into the large rhinencephalon, figs. 73 and 46, R, d.

The characteristics of the Marsupial brain are, its small relative size, small proportion of cerebrum, convolutions wanting, or few and symmetrical in those Marsupials possessing them, large proportional anterior commissure, and still larger hippocampi; some fibres arch across from one to the other hippocampus, answering to the lyra,' and forming the beginning of the great transverse commissure or corpus callosum' of higher Mammals; the fibres radiating upon the fore and inner wall of the ventricle are the anterior terminations of the great longitudinal commissure answering to the fornix' in anthropotomy. The corpus striatum,' fig. 75, r, is relatively small and inferior in position to the hippocampus, being partially overlapped thereby.

6

B. Lissencephala.—I demonstrated the characters differentiating the first step in the developement of the corpus callosum' of the Hedgehog, in the longitudinal section of the brain1 prepared and added to the Hunterian series of Comparative Anatomy in 1834, by contrast with a similar section of the brain of the Opossum and Dasyure; placing a plate of mica in the fore part of

Haller showed his appreciation of the essential nature of these fere fornicis ipsius cruribus.' 8 Ib. lc.

2 LXX'. pl. v. fig. 8, la.

4 No. 1323 D, xx. vol. iii. (1835), p. 29; and see XLIII". pl. xxxvii. fig. 7.

5

XX. no. 1323 B.

6

XX. no. 1323 C.

6

the hippocampal fissure in each, which accordingly passes above the transverse commissural system (lyra of fornix') in the Marsupial, and beneath the abruptly superadded corpus callosum in the placental Insectivore. In a similar section of the brain of the Squirrel' the corpus callosum is of greater relative extent, as it is in all Rodents as contrasted with Insectivores. Concomitantly with the appearance of the new series of transverse fibres bringing the hemispheres into communication above their ventricles, the anterior commissure is diminished in size.

Notwithstanding this difference in the kind and arrangement of the transverse connecting fibres of the hemispheres, these do not present a corresponding rise of developement. In the snouted Shrews of Africa the brain, fig. 76, offers outwardly as low a condition as in the Opossum or Dasyure. All the four primary segments are in view; the epencephalon, C, mesencephalon, o, prosencephalon, P, and rhinencephalon, R, succeed each other longitudinally from behind forward, as in Reptilia. The multiplication of grey and white matter above the medulla oblongata mainly distinguishes the brain of the active Shrew from that of the slow Tortoise, fig. 45; and the lateral lobes of the cerebellum carry appendages, as in the Opos

sum.

76

Brain of Rhynchocyon

LXXXIV.

The anterior bigeminal bodies, o, much exceed the posterior ones in size. A feeble and interrupted indication of the medilateral longitudinal fissure marks the upper surface of the hemispheres. These are much contracted anteriorly. A short callosal fissure is added to the hippocampal one on the inner surface of the hemisphere. The rhinencephala are long, large, and pyriform. In the Hedgehog (Erinaceus) the ectorhinal fissure is apparent in the upper view of the brain through the great relative size of the crura rhinencephali.

The Bats resemble the terrestrial Insectivora in their cerebral surface, as do also the smaller Rodents. In some of the larger ones, Agouti, e. g. (vol. ii. p. 270, fig. 146), the medilateral fold is better defined: but the Beaver shows no trace of this, although the hemispheres are broader anteriorly: they are more expanded here in the equally smooth cerebrum of the Porcupine, fig. 77. The Rodents show some variety in the shape of