lateral column are marked p, those continued over the olives, w, and those over the prepyramids, v; they form the trapezium in lower Mammals.

The nucleus in the trapezium, on each side of the raphe, so closely resembles, at a higher section, the olivary body, that it has

Magnified ten diameters.

been termed the upper olive'; it makes its appearance near where the lower olives first diminish in size. In the Sheep it appears as a group of large stellate multipolar cells, and these cells are more numerous in the Rodents, and still more so in the Cat. In the Rabbit the upper olivary body is convoluted in three or four turns; in the Mouse it consists of a wavy mass of large and numerous cells; its structure is especially distinct in the Cat.

The post-pyramidal' and 'restiform' nuclei are present in all Mammals. The olivary bodies consist of layers of small cells penetrated by the arciform filaments, by which they are connected with each other and with the raphe; they are not absent in the Sheep. The transverse section of the human medulla oblongata in the region of the first cervical nerve is more circular, less

elliptical, than in the Sheep and most lower Mammals. The restiform and post pyramidal nuclei are relatively larger, but the Quadrumana and Carnivora approach the human structure in this particular; the Cat, e. g., shows an intermediate condition between those in Ruminantia and Bimana.1

In comparing the macromyelon of the Mammal (fig. 50) and Fish (vol. i. fig. 172) the usual course of structural differentiation seems to be reversed; a greater number of longitudinal tracts are definable in that of the Sturgeon or Shark than in that of Man. But the superior character is more seeming than real; the superaddition of ascending fibres in the higher Vertebrate tends to obliterate the boundary lines and seems to blend tracts--the 'funicular' and post-pyramidal, e. g. in the Mammal, which are distinguishable in the Fish.

§ 206. Cerebellum.-The posterior and restiform columns, pushed aside by the postpyramidal and teretial tracts in approaching the macromyelon, diverge and expand into a fibrous stem, which, arching over the fourth ventricle, developes the central transversely folded lobe, answering to the cerebellum of the Shark (vol. i. fig. 187, c) and Bird, and expands into lateral lobes

Vertical section of the median lobe of Cerebellum and Macromyelon.

characteristic of the Mammalian class. The myelonal tracts, which in describing the brain from behind forward may be said to enter into the formation of the cerebellum, fig. 66, r, leave it, after some expenditure and exchange of substance, as 'departing'

The progress of chemistry has lent new and valuable aids to the unravelling of the minute, but physiologically most interesting, structures of the myelon and macromyelon. A solution of chromic acid is one of the best for preliminary immersion of slices of their tissues for a few weeks; these, if afterwards put into alcohol, are hardened, but become less brittle than if kept longer in the acid.

restiform tracts, ib. t, continued into the basis of the mesencephalon, forming also those called 'processus cerebelli ad testes,' united above by the thin layer of medullary matter called ' valve of Vieussens,' fig. 49, B. The progressive increase of the lateral lobes is attended by corresponding developement of the system of transverse or arciform fibres constituting the 'pons varolii,' which, entering the cerebellum at the infero-lateral' or 'semilunar fissure,' fig. 64, h, i, interblend with the longitudinal entering' and 'departing' columns, and constitute the commissural part of these lobes.

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In Anthropotomy the part where the formative and commissural tracts join on entering the cerebellum are collectively called its crus,' the tracts being its constituent peduncles; ' thus the entering or posterior and restiform tracts, which are the homotypes' of the crura cerebri,' are termed the inferior or posterior peduncles,' or 'processus ad medullam oblongatam,' fig. 66, r; the emerging restiform tracts, called 'processus ad cerebrum,' and 'processus ad testes,' are the 'superior or anterior peduncles,' ib. t; whilst the entering fasciculi of the 'pontal or varolian commissure' are the middle peduncles' or 'processus ad pontem,' fig. 64, i.

These latter are porportionally least in the lowest, and largest in the highest, species of Mammals. In all, the formative columns on entering the white axis receive grey or recruiting' matter for the developement of accessory fibres, relating in size and complexity to the increase of the cerebellum, and chiefly of its lateral lobes. In the Monotremes, figs. 51 and 52, the pontal' or cerebellar commissure is a thin layer of transverse fibres of small antero-posterior extent; the true character of the real crura cerebelli,' or formative fasciculi, is here well exemplified. The cerebellum, fig. 38, b (Echidna), consists mainly of the median lobe, which being transversely folded presents in vertical section that arrangement of grey and white matter called arbor vitæ.'

In the Marsupial Order, the cerebellum presents close-set, subparallel, transverse convolutions; few in the climbing Koalas and Opossums, fig. 46, c, more numerous in the locomotive Kangaroos: it is remarkable, as in Monotremes, for the large proportional size of the median or vermiform lobe as compared with the lateral lobes, especially in the carnivorous and insectivorous Marsupials, where this condition is associated with a corresponding diminution of their commissural band as shown in the view of the base of the brain of an Opossum, fig. 53, b. In the Kangaroos,

Perameles, Phalangers, and Koala, the hemispheres or lateral lobes of the cerebellum are characterised by a small subspherical lateral process or appendage, c, c, fig. 74, which is lodged in a peculiar fossa of the petrosal above the internal meatus: there are corresponding but less produced processes in the Dasyures and Opossums, they do not project in the Wombat. On the upper surface of the cerebellum the medullary substance or nucleus appears superficially at a small tract on each side the vermiform process, marked with an asterisk in figures 74 and 75.1 The simple disposition of the arbor vitæ is shown in fig. 46, e.

In the Lissencephala, the cerebellum in the Insectivora, fig. 76, and Cheiroptera, resembles that of the Opossums; in the Rodentia the lateral lobes, fig. 59, d, show a greater increase, which is most marked in the swift running Hares, fig. 81, 1, 1. As this developement is not accompanied with a concomitant growth of the cerebrum, the cerebellum is proportionally greater to the rest of the brain in Rodents than in other mammalian orders.

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The Cetacean brain is remarkable for the large proportional size of the cerebellum, fig. 60, and especially of its lateral lobes, c. On the under surface may be distinguished the main part of the lateral lobe, e, the oblique lobule, f, that which answers to the amygdaloid lobe' and the floccus of Reil, h. Each is subdivided by the chiefly transverse anfractuosities into numerous lamellæ. The middle lobe, fig. 93, a, is not symmetrical but inclined, like the skull, to one side, in Delphinus. The grey nucleus or corpus fimbriatum' is well developed. The 'pons,' fig. 60, c, is now large and prominent.

In the Ungulata, the relative size of the lateral lobes increases with the bulk of the species, and attains its maximum in the Elephant; in the Rhinoceros, Giraffe, fig. 86, Or, and Horse, fig. 61, the middle lobe is contorted, especially above. The common castration of the latter quadruped has afforded abundant evidence that the cerebellum is in no degree affected thereby in size or form.2

In Carnivora the smallest species (fig. 89, Stoat) have the smallest lateral lobes in proportion to the middle one.

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In the Quadrumana the middle lobe is proportionally largest in the Lemuride and smaller Platyrhines. The appendicular lobule is present in the Aye-aye' and other Lemurida, and is lodged in a special pit of the petrosal. In the larger Catarhines the lateral lobes increase in size,

and lose, or incorporate, the appendix; they show the lobular groups of lamellæ, especially on the under surface, fig. 62, as

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in Cetacea, and in Man. The flocculus,' fig. 64, n, to which the origin of the acoustic nerve can be traced, is present in all Quadrumana, and is well marked in

Brain of the Horse.

cu. p. 56, fig. 3, 3.