others at an acute angle, several laminæ, each of which forms the stem of a number of other branches. Each of the primary branches is the foundation or central stem of a lobule. Lamina of fibrous matter are seen branching from both sides of it immediately after its separation from the nucleus. Sometimes the primary branch bifurcates, and each division of it forms the stem of what may be called a sub-lobule. If we suppose that one of the primary branches is composed of a certain number of lamina

Dissection of the formative columns of the ep-, mes- and pros-encephalon.

of fibrous matter, the secondary ramifications from it will in a great degree correspond. In most instances these secondary branches subdivide into two or more tertiary ones, which, as well as the branch from which they spring, are enclosed in grey matter. A vertical section of the median lobe, fig. 58, gives a similar appearance to that of the hemispheres, fig. 66, c. The central

nucleus breaks up into primary branches, which become the centres of the lobules of which it consists. The ramifications of the nucleus, whether of the median lobe or of the hemispheres, pass from it only in the vertical plane or from before backwards; in the latter direction, however, to a very slight extent. The fibrous matter of the median lobe is continuous with that of the hemispheric lobules. By reason of this disposition of the fibrous matter, the surface exposed by a horizontal section through the entire cerebellum consists of a plane of white matter bounded on the sides and behind by a narrow cortex of grey matter.

The white matter consists exclusively of fibres, chiefly of the tubular kind, and of all degrees of size. These, in the more distant ramifications, penetrate the vesicular matter of their grey cortex, and form some unknown connection with its elements. The grey matter consists of three layers, readily distinguishable by the naked eye from their difference of colour. The external layer is the darkest, and consists chiefly of granular and vesicular matter. The next or intermediate layer is of a light colour, and is composed of a stratum of fine nucleus-like particles. The third layer has the greatest thickness, and is immediately in contact with the fibrous matter; it is intermediate in point of colour to the other two, and consists of numerous vesicles of the caudate kind, especially with branching processes and nerve-tubes of all sizes. The dark colour of the external layer is doubtless owing in a great measure to the great numbers of capillary vessels which enter it; the greater paleness of the inner stratum is to be attributed to the intermixture of the white fibres, whilst the light colour of the middle stratum is intrinsic.'

The lower or hinder crus, fig. 66, r, on entering the cerebellum curves backward, expanding on the outer side of the converging and onwardly continued fibres which constitute the upper or 'anterior crus,' t. In the part of the nucleus' connected with the latter is developed a plicated capsule of grey or vesicular matter, d, also exposed in section at R, fig. 49, and called corpus dentatum;' it supplies accessory white fibres to those diverging from, or converging to, the crura; with these are interlaced the commissural fibres of the pons.

Thus an influence ascending from the myelon, by the restiform tracts, fig. 66, s, r, to the cerebellum, may be propagated from that body, by the crus, t. to the mesencephalon, and thence to the cerebrum. Conversely, cerebral influence may pass through the mesencephalon by the processus and

XXVII". p. 692.

testes,' ib. t, fig. 66, to the cerebellum, and thence by the restiform tracts, ib. r, to the myelon, s: while the transverse fibres of the pons, ib. v, associate all the parts of one cerebellar hemisphere in action with the other, and are intimately connected and interlaced with the longitudinal fasciculi forming the crura cerebri.

If it be considered that the maintenance of the erect position by Man demands unusual power of regulating and combining muscular movements, whether with or without the cognisance of the mind, and that he exercises or can exercise a greater variety of modes of locomotion than any lower animal, flight alone being inexecutable, the characteristic size and complexity of the human cerebellum would accord with such view of its functions; and the general results of the experiments of Flourens' and Majendie2 concur with the inferences which, in the main, may be drawn from comparative anatomy (vol. i. p. 287).

§ 207. Mesencephalon.-Part of the columnar fibres continued from the epencephalon proceed directly to the prosencephalon, traversing the pons, fig. 66, p, v. The olivary tracts, ib. f, proceed first to the mesencephalon, which likewise receives the crus, t, or continuation of the restiform tract, r, after having undergone cerebellar developement and connections.

The mesencephalic basis is traversed by a forward continuation. of the primitive myelonal cavity-the 'iter a quarto ad tertium ventriculum'—which latter, fig. 105, b, is a vertical expansion of the 'iter,' extending upward into the pedicle of the conarium (‘pineal gland,' ib. f), and downward into that (infundibulum ') of the hypophysis ('pituitary gland,' ib. v). The sides of this ventricular fissure are partially glued together by grey matter continuous with that in the interior of the thalami,' and called 'soft commissure' in front of b, fig. 105.

In the Monotremes the mesencephalic crus ('processus a cerebello ad testes '), receiving a tract, answering to the 'fillet' of anthropotomy, expands into the optic lobe ('nates,' ib.), forming chiefly its exterior white layer: the primitive cavity of this vesicle becomes filled with grey matter. The layer (valvula') uniting the two crura becomes thickened by transverse white fibres behind the optic lobes, and these, in higher mammals, swell into a second pair of tubercles ('testes,' ib.), which usually exceed the nates' in breadth, but are less in length; they now form the corpora bigemina, or quadrigemina' of anthropotomy. The above difference in the proportion of the

2 LVI".

1 LV". and LXIV.

VOL. III.

H

67

B,

two pairs is exemplified in fig. 73, b, Didelphys, and fig. 75, Phascolomys, in the Marsupial order; by fig. 79, Lepus, and fig. 80, 8,9, Cavia, in the Rodentia; and by fig. 67, Talpa, in the Insectivora. Both Ly- and Liss-encephala manifest their inferior position in the present class, and affinity to oviparous Vertebrates, by the larger proportion of the mesencephalon (fig. 46, o) to the prosencephalon, than in Gyrencephala. In most Marsupials (Dasyurus, fig. 72; Didelphys, fig. 73), in many Rodents (fig. 81, Lepus; fig. 80, Castor), in all Insectivores (fig. 76, Rhynchocyon), and in Bats, the bigeminal bodies are more or less exposed between the cerebrum and cerebellum. As in Amblyopsis (vol. i. p. 278, fig. 175), so in Talpa, the optic lobes, fig. 67, c, do not show a reduction of bulk commensurate with that of the visual organ; yet there is a degree of such relationship in Mammals. Thus the Ungulates which have large eyes have the optic lobes or nates, fig. 68, a, proportionally larger than they are in a Carnivorous quadruped with a similar-sized brain. In both the testes,' ib. b, are broader, but in Felis they also rise higher; whilst in Ungulates, and especially Ruminants, the 'nates' show the greater vertical developement. In all Carnivores the 'testes' have a

Brain of Mole.

68

minor antero-posterior extent than the 'nates.' The white bands or tracts (brachia' in anthropotomy), extending along the outer sides of the bigeminal bodies to the thalami and commencement of the optic tracts, fig. 68, d, are prominent in the higher Quadrumana and in Man. In most Gyrencephala the white fibres

continued from the optic lobes develope an oblong nodule, ib. e, also containing grey matter (corpus geniculatum' of anthropotomy), which in the human brain is divided into an external and internal portion.

The crura cerebri' formed by the pre- and post-pyramidal

1 This difference I exemplified in the preparations, nos. 1326 A and 1326 B, XX. vol. iii. p. 30.

and teretial' tracts, expand in passing beneath the bigeminal bodies, and receive accessions from grey matter continuous with that of the macromyelon, but so dark as to have received the name locus niger' when exposed in section. They are divided by the third ventricle, and swell out respectively at their upper part, through the superaddition of formative neurine, into the bodies called 'thalami optici,' fig. 68, c, figs. 71 and 75, t. The free surface is white, but the grey matter constitutes their chief bulk, and is partially divided by the longitudinal fibres into an outer and an inner portion: from the latter the soft commissure is continued. The optic tracts, fig. 68, d, commencing at the optic lobes and geniculate bodies, bend round the outer and back part of the thalami,' from which they derive accessory filaments to form the optic nerve. In connection with the mesencephalon must be noted the tract of white fibres continued from the fornix, on each side the third ventricle anterior to the soft commissure, to a nodule, conspicuous in Gyrencephala behind the infundibulum, and forming a pair ('corpora albicantia' in anthropotomy) in Apes, fig. 112, and Man.

6

§ 208. Prosencephalon.-As the 'crura cerebri' enter the prosencephalon, they are augmented by further accessions of formative neurine in masses which in the human brain have received the names nucleus tæniæformis,' nucleus lenticularis,' and nucleus. caudatus.' The latter projects into the prosencephalic ventricle, as the corpus striatum,' figs. 70, s, 75, r. But this name extends or applies also to the deeper-seated grey masses, which are so interblended with the diverging white fibres as, in section, to give alternate white and grey striæ. The accession of white fibres from these formative nidi, diverging to form the basis of the cerebral hemispheres, causes the form expressed by the term 'fibrous cone,' fig. 66, c. The grey matter again appears as a thin superficial covering or cortex' of the expansion of the white fibres and this grey matter contains cells similar to those in the corpus striatum.

In most Ly- and Liss-encephala, and in a few of the smallest kinds of Gyrencephala, the prosencephalic vesicles retain the outward uniformity of surface which they have in birds and reptiles : unlike those of the mes- and ep-encephalon, they are so little united together that they are called and seem to form distinct ' hemispheres.' These are connected together in all Mammals as in Birds by the cord-like fasciculus of transverse fibres, figs. 69 and 73, c, called anterior commissure.' But the main distinction lies in the superaddition to the diverging' or 'crural'