fibres of other commissural' tracts either longitudinal,' connecting parts of the same hemisphere, or transverse,' and bringing a greater proportion of the two hemispheres into mutual communication. But there are steps in this differentiation.

6

Each hemisphere of the cerebrum begins as a vesicle of neurine, the cavity of which receives the growth from the crura' forming the corpus striatum.' This, in Birds, mainly fills the 'ventricle' or remnant of the primitive cavity of the sac. But, in Mammals, the wall of the vesicle is augmented by folds, of which the first and most constant is pushed from the mesial or inner side of the ventricle into its cavity, giving rise to the convexity, figs. 70, 71, h, fig. 75, n, representing the part called hippocampus' in anthropotomy. The fissure upon which the hippocampus is folded' is numbered 4 in the Table of Cerebral Fissures,' p. 136, as in fig. 69, et seq.

1

In Lyencephala it extends from the fore part of the inner surface of the hemisphere backward and downward in a curve with the concavity toward the centre or nucleus cerebri,' fig. 69, b. It is not, however, a mere doubling of the wall of the hemispheral

R

69

Inner surface of hemisphere. Vertical section of brain, Ornithorhynchus.

vesicle; longitudinal fibres are developed therein for commissural office; they cause a definite production of the lower part of the fold within the ventricular cavity called hippocampal band (tania hippocampi), or, because in Man it is plaited, corpus fimbriatum:' its inferior hinder termination is in the 'pes hippocampi;' its upper or anterior one becomes the 'posterior pillar' of the fornix. Fornix' is the anthropotomical term for the anteriorly continued and transversely connected longitudinal fibres of the hippocamp: the posterior pillars,' fig. 69, a, one from each hemisphere, converge as they advance, are united by a commissure of their own, ib. o, beyond which some fibres pass forward and radiate upon the inner surface of the fore part of the hemisphere; while others bend down, as the anterior pillars' of the fornix, pass between the anterior commissure, ib. c, and the nucleus cerebri, b, and terminate in the mammillary body already mentioned.

Delicate fibres, running on the inner surface of the hemisphere at right angles to the line of the hippocampal fissure, are continued into the ventricle, where they cover the longitudinal fibres

So defined in LXX'. p. 90 (1837)

developed in the hippocampal fold, and which form the main part of the hippocamp and its anterior extension.1

This fold and its concomitantly developed longitudinal and transverse or arched fibres, constitute a great and abrupt distinction and rise in structure in the Mammalian brain as compared with the Avian one, and indicate that birds are an offshoot from the lower Ovipara, forming a branch apart.2

In Ornithorhynchus the postero-inferior parts of the hemispheres are brought into connection with the antero-internal parts by the longitudinal fibres, while the antero-internal parts of the hemispheres are connected with each other through the transverse fibres at the approximated anterior ends of the folds, where the stratum connecting those ends together, and radiating the fibres upon the inner surface of the anterior lobes of the hemispheres, and over the inner wall of the ventricle, is thickest.3

The greater part of the hemispheral cavity or ventricle is overarched in Lyencephala by the inner leaf of the hippocampal fold, and its developements called 'tania hippocampi' and 'fornix.' The transverse fibres connecting the tænia hippocampi and terminating that body anteriorly in Lyencephala, are carried, in the ascending Mammalian series, by the growth of the hemispheres anterior to them, as it were by a movement of rotation, from before upward and backward, until, in Man, they become the psalterial fibres' which connect the posterior 'genu' of the corpus callosum with the tænia hippocampi,' these being compared to the frame' and the transverse fibres to the 'strings' of the harp, by the old anthropotomists. The superaddition of cerebral matter above and anterior to c, figs. 69, 73, is associated with transverse commissural fasciculi, progressively added, from behind forward, and now overarching the lateral ventricles, and fulfilling all the functions, relations, and definitions of the anthropotomical corpus callosum,' figs. 78, 1, and 123, c. Its hind part is embraced by the callosal convolution,' ib. o.a

'These fibres are shown at r, fig. 4, pl. vii. LXX'., which gives a view of the hippocampal fold from the ventricular or 'lateral' side, as part of a thin stratum of medullary fibres arching over the hippocampus major, and continued therefrom into the internal wall of the ventricle,' p. 95.

* If we could examine the brains of Dinosauria or Dicynodontia, the actual gap in the series of cerebral structures might be better filled.

* From this point in the lowest (Lyencephalous) mammals, as in the embryo of the highest, the growth of the great supraventricular body of transverse commissural fibres forming the corpus callosum' begins: Anterior fibres of the "tænia hippocampi" continued into the anterior lobes of the hemispheres.' LXX'. p. 95, pl. vi, figs. 4 and 6, d'; and pl. vii. fig. 4, x.

The part marked в in the Echidna has become the part marked N in Man. Pls. xxxvi. and xxxviii, of XLIII”.

Such are the essential characters of the Mammalian prosen-cephalon.' The chief modifications of the Mammalian brain, as above characterised, will next be noticed in the different leading groups of the class.

70

A. Lyencephala. In the Ornithorhynchus, the brain, figs. 52 and 69, is to the weight of the body as 1 to 130; the hemispheres are triangular, depressed, the broader posterior part overlapping the optic lobes, and reaching to the cerebellum. With the exception of the hippocampal fissure, fig. 69, 4, and the depression lodging the rhinencephalic crus, the surface is unbroken or smooth, with a few vascular impressions diverging from the fore part. The medulla oblongata is broad and depressed; the corpora pyramidalia, fig. 51, a, are in very low relief; the corpora olivaria, a', expand as they advance; they are crossed anteriorly by the corpora trapezoidea,' b, which are large; the 'pons,' c, is narrow anterior to it is a large ganglionic body, c', from which issues the huge trigeminal nerve, 5. The longitudinal groove between the optic lobes is shallow;

71

it is wanting in the small and low testes.' The hippocampus is the chief prominence within the ventricle of the hemisphere; the corpus striatum is long and

narrow.

The brain of the Echidna, fig. 71, is relatively larger than in the Ornithorhynchus, and the exposed outer surface of the hemispheres is extended by convolutions. The cerebral hemispheric cavity is mainly occupied in both Monotremes by the hippocamp,' fig. 70, h, which constitutes a great part of its floor as well as inner wall. This, with much of the hippocamp, is removed in fig. 71, to show the proportions of the corpus striatum,' s, and to bring into view the thalami, t; these are divided from thenates,' r, by a linear groove; the testes,' s, are half the size of the nates,' and the median longitudinal groove, which is shallow between the nates, is not continued further

Echidna.

6

back. Like the water-shrews, the Ornithorhynchus has a smooth cerebrum; the Echidna, like the Great Ant-eaters and the Sloths, has a convoluted one. Besides the long and deep hippocampal fold,' the fore part of the mesial surface shows a beginning of the supercallosal one; behind which it is also notched vertically by the mesial ends of the upper transverse folds, fig. 71. Of these, three nearly parallel ones extend across the broad posterior part of the upper surface of each hemisphere, their outer ends inclined forward; anterior to them is a larger convolution bent upon itself so as to form the inner boundary of the anterior half of the upper surface. In the angle of the above are two oblique folds inclining 'mesiad' toward the contracted fore part of the hemisphere. The base of the brain, fig. 52, shows a few short foldings of the surface of the great natiform protuberances, b'. The principal folds sink about a line's depth into the substance of the cerebrum. The rhinencephalon is enormous, ib. R. Some of the fibres of the great anterior commissure bend forward, and are continued into each of its crura. The outer part of the crus, ib. 1a, continued from that of the prosencephalon, emerges from the fore margin of the natiform protuberance, from which it has a reinforcement of fibres; the inner division, tumid with added grey neurine, ib. 16, is also very broad. The prosencephalic cavity or ventricle' is continued into the rhinencephalon, and is exposed in fig. 52, by removal of the thin floor which rests upon the large cribriform plate.' The 'pineal' and pituitary (ib. p) appendages of the prosencephalon offer no monotrematous characters.

There is not that difference of size between the Ornithorhynchus and Echidna which would lead us to connect therewith the convolution of the hemispheres in the latter animal; what is known of their habits suggests no superiority of psychical power and resource in the land- over the water-monotrematous Insectivore. Increased extent of the walls of the hemisphere in no

'My observations on this state of the corpora quadrigemina' in Monotremes accord with those of Laurent and Eydoux on the Echidna, and of Meckel on the Ornithorhynchus. En comparant les tubercules quadrijumeaux de l'Échidné à ceux de l'Ornithorhynque, nous avons facilement constaté ce que l'a déjà été par Meckel pour ce dernier, c'est-à-dire qu'on ne peut pas distinguer les tubercules postérieurs des antérieurs, et que ce que Meckel a remarqué chez l'Ornithorhynque et exprimé en ces termes : “Eminentia quadrigemina magna, posterior tamen verè percipienda, ut ferè bigemina esset," est encore plus prononcé dans les tubercules du cerveau de l'Échidné, qui sont réellement bijumeaux simplement.' LVII". p. 164.

2 Well given in LVII". pl. ix. fig. 4: omitted in the diagram of a similar section in XLIII". pl. xxxvii. fig. 7.

degree influences the developement of a supraventricular transverse commissure; the seeming small one exposed at o, fig. 71, is hippocampal or psalterial. This low phase of Mammalian braingrowth is essentially related to the common monotrematous conditions of generation.

The brain bears a small proportion to the body in the Marsupial order; in the Ursine Dasyure, fig. 72, it is as 1 to 520; in the Wombat, as 1 to 614; in the great Kangaroo, as 1 to 800. In smaller Kangaroos the disproportion is less; thus in the Treekangaroo (Dendrolagus inustus) I found it as 1 to 250. The brain is relatively largest in the smaller species of Petaurists and Phalangers.

The cerebral hemispheres do not extend over the cerebellum in any of the species, and in some, as the Dasyures and Opossums, they leave the optic lobes exposed. In the Phalangers and Petaurists, the Opossums, Perameles, the insectivorous Phascogales, and the smaller Dasyures, the exposed surface of the cerebral hemispheres is unconvoluted. In the Dasyurus ursinus, fig. 72, b, this surface is broken by a few slight indentations, two of which may indicate the beginnings of the 'medi-lateral' longitudinal folds.

72

1

6

In the Wombat an ectorhinal fissure bounds the outer side of the olfactory tract at the base of the brain; from the anterior moiety of this fissure three or four smaller ones curve upward upon the sides of the hemispheres, one of which answers to the fissura Sylvii,' but is less defined than in the Kangaroo. On the upper surface a short transverse fissure marks off the outer part of the anterior lobe of the cerebrum, and behind this each hemisphere exhibits a few detached shallow fissures.

2

The American Opossums show a range in size from that of a mouse to that of a cat, and the Australian Dasyures rise from the same diminutive extreme (Antechinus pusillus) to the size of the wolf (Thylacinus). But the cerebral hemispheres are as smooth in Didelphys Virginiana3 as in D. (Philander, Microdelphys) murina; and the great Ursine Dasyure, fig. 72, shows but a few short and shallow indentations of the exposed cerebral surface. Thylacinus Ib. fig. 5.

1 LXX' pl. v. fig. 8.

3 Ib. fig. 6.

2 Ib. fig. 3.