of the cerebellar lamellæ (fig. 70, A, B). But the further development of these cells has not as yet been ascertained, and it can only be conjectured that they become gradually transformed into cells of the molecular layer, for no such superficial zone can be seen in the adult cerebellum. At early stages of development there are also to be seen in the molecular layer, bipolar cells placed with their axes vertical, and having protoplasmic processes extending down towards or into the rust-coloured layer, and axis-cylinder processes extending towards the surface, and becoming continued by T-shaped junctions into fibres running parallel with the processes of the horizontal bipolar cells which have just been described. These vertical bipolar cells may perhaps be derived from the horizontal ones, and represent a stage in the formation of the smaller granules of the rust-coloured layer, but we have no clear evidence as to their further course of development. Like the superficial granules they also are only found in young animals. The cells of Purkinje in the cerebellum of embryonic and very young animals are very irregular multipolar cells, with relatively short protoplasmic processes, but a welldeveloped axis-cylinder process, which already has two or three collateral fibres extending towards and ramifying in the deeper part of the molecular layer (fig. 69). It is not until two or three weeks after birth (in the dog) that the dendrites begin to assume the characteristic form and arrangement. The arborescence which in the adult cerebellum envelopes the principal dendrites of the cells of Purkinje (see p. 91 and fig. 67), is at this early stage chiefly confined to the upper part of the body of the cell (fig. 70). DEGENERATIONS FOLLOWING CEREBELLAR LESIONS. The degenerations of nerve-fibres which follow lesions of the cerebellum have been investigated by Marchi in animals (dogs and monkeys) operated upon by Luciani. Hemiextirpation (see fig. 71).-After removal of one half of the organ extensive degeneration is seen in all three peduncles of the same side; very little, if any, in the peduncles of the opposite side. It may therefore be inferred that none of the peduncles contain commissural fibres connecting the two halves of the organ. The degenerated fibres in the superior peduncle pass partly to the tegmental nucleus of the opposite side, and partly to that of the same side; in other words, the decussation of these peduncles in the region of the inferior corpora quadrigemina is not complete. Some fibres can be traced as far as the optic thalamus. The middle peduncle, after removal of the corresponding half of the cerebellum, is completely degenerated as far as the raphe. Degenerated fibres are seen intermingled with the pyramidal fibres both of the same side and of the opposite side, and with the fibres of the fillet and posterior longitudinal bundle, mainly of the same side. Degeneration and atrophy are also produced in the grey matter of the pons (nuclei pontis) of the same side. The degeneration in the fillet and posterior longitudinal bundle is most marked on the side of the lesion; it can be followed upwards to the region of the corpora quadrigemina, and downwards to a tract at the periphery of the antero-lateral column of the spinal cord. A bundle of degenerated fibres is also seen passing to the pyramidal tract. These appear to pass upwards towards the corpora quadrigemina, and probably to the corpus striatum, chiefly of the same side; a few are traceable downwards into the cord. The inferior peduncle, after hemiextirpation of the cerebellum, is notably degenerated in its inner and outer parts. A small degenerated bundle is traceable with the inner arched fibres, across the raphe to the opposite lower olive which undergoes complete atrophy; other arched bundles pass from the restiform body to the tract of the fillet, and of the posterior longitudinal bundle of the same side; and from these, as above stated, many degenerated fibres are traceable down the periphery of the antero-lateral column of the cord, some down the pyramidal tract, but most are situated ventral to the direct cerebellar tract of Flechsig in the antero-lateral descending cerebellar tract. It is also stated by Marchi that after hemiextirpation of the cerebellum, degenerated fibres are seen passing along with the roots of nearly all the cranial nerves, especially the second, third, fifth (ascending root), sixth, seventh, and twelfth, and the striæ medullares, and along with the anterior roots of the spinal nerves. These degenerated fibres are numerous on the same side as the lesion, but a few occur in the opposite nerves. Those which pass to the cranial nerves run along the posterior Fig. 71.-OUTLINE OF SECTIONS SHOWING THE DEGENERATIONS FOLLOWING EXTIRPATION OF THE LEFT HALF OF THE CEREBELLUM IN THE DOG. (Marchi.) A, lumbar cord; B, cervical cord; C, medulla oblongata; D, pons Varolii ; E, mid-brain at nucleus of third nerve. In A and B, the degeneration is in the antero-lateral column of the same side as the lesion, except in B, where there is a little degeneration on the opposite side. In C, a indicates the restiform body; b, the ascending root of the fifth; c, the posterior longitudinal bundle; d, the antero-lateral tract; e, the pyramids; f, the olivary nucleus; g, the fillet; h, the hypoglossal nucleus. In D, a is the superior cerebellar peduncle; b, the middle peduncle; c, the posterior longitudinal bundle; d, the fillet; e, the antero-lateral tract; f, the raphe. In E, a is the nucleus tegmenti (most degeneration in the crossed superior cerebellar peduncle); b. the issuing fibres of the third nerve; d, the posterior longitudinal bundle; e, the crusta; f, part of the fillet. longitudinal bundle, those which pass to the spinal nerves down the descending antero-lateral tract of the cord. Extirpation of worm.-After hemi-extirpation of the middle lobe only of the cerebellum the degeneration in the superior peduncle is comparatively slight, and entirely crosses at the decussation to pass to the tegmental nucleus of the opposite side. The degeneration of the middle peduncle is most marked of the pons; but little occurring in the lower two-thirds. in the upper third That of the inferior peduncle is limited to the outer or lateral part of the restiform body. A few fibres pass from this across the raphe to the lower olive of the opposite side. Others pass as arched fibres to the tract of the fillet, to the posterior longitudinal bundle, and through this to the cranial nerve-roots, especially the third, fifth, eighth, and twelfth. Others are traceable down the antero-lateral columns of the cord, but those to the pyramidal tract are lacking. RECENT LITERATURE OF THE CEREBELLUM. Bechterew, W., Zur Anatomie der Schenkel des Kleinhirns, insbesondere der Brückenarme. Neurolog. Centralbl., 1885; Ueber die Bestandtheile des vorderen Kleinhirnstiels, Neurolog. Centralbl., 1887, and Arch. f. Anat. u. Phys., Anat. Abth., 1888. Beevor, Ch. E., Die Kleinhirnrinde, Arch. f. Anat. u. Physiol., 1883. Bellonci, G. e Stefani, A., Contribuzione all' istogenesi della corteccia cerebellare, Memoria letta all' Accademia de Ferrara, 1886: Archives italiennes de biologie, t. xi, 1889. Borgherini, Contribution à l'histologie normale du cervelet, Archives ital. de biol., xii, 1889. Brosset, J., Contribution à l'étude des connexions du cervelet, Thèse, Lyon, 1890. Cramer, A., Einseitige Kleinhirnatrophie, &c., nebst einem Beitrag z. Anatomie der Kleinhirnstiele, Beiträge zur pathol. Anatomie, Bd. xi., 1891. Fusari, Sull' origine delle fibre nervose nello strato molecolare delle circonvoluzioni cerebellari dell' uomo, Atti della R. accad. delle scienze di Torino, 1883. v. Gehuchten, A., La structure des centres nerveux, La Cellule, t. vii, 1891. Golgi, C., Recherches sur l'histologie des centres nerveux, Arch. ital. de biologie, t. iii. et iv., 1882. v. Gudden, P., Ueber d. Verbindungsbahnen des Kleinhirns, Berichte d. deutsch. Natürf.-Versammlung, 1882 (Neurol. Centralbl.). v. Kölliker, A., Das Kleinhirn, Zeitschr. f. wissensch. Zoologie, Bd. 49, 1890. Luciani, L., Il cervelletto, Firenze, 1891. Marchi, V., Des dégénérations consécutives à l'extirpation totale et partielle du cervelet, Archives italiennes de biologie, t. vii, 1886. Mendel, E., Secundäre Degeneration im Bindearm, Neurol. Centralbl., 1882. Mingazzini, G., Intorno al decorso d. fibre appart. al pedunculus medius cerebelli ed al corpus restiforme, Arch. p. 1. scienze mediche, 1890. Obersteiner, H., Der feinere Bau der Kleinhirnrinde, Biol. Centralbl., Bd. iii., 1885. Ramón y Cajal, S., Sobre las fibras nerviosas de la capa molecular del cerebelo, Revista trimestrial de histologia normal y patologica, 1888 and 1889; Sur l'origine et la direction des prolongations nerveuses de la couche moléculaire du cervelet, Internationale Monatsschr. f. Anat. u. Phys., Bd. vi, 1889; Sur les fibres nerveuses de la couche granuleuse du cervelet et sur l'évolution des éléments cérébelleux, Ibid., Bd. vii, 1890; Apropos de certains éléments bipolaires du cervelet avec quelques détails nouveaux sur l'évolution des fibres cérébelleuses, Ibid., vol. vii, 1890. Retzius, G., Die nervösen Elemente der Kleinhirnrinde, Biol. Untersuch. Neue Folge, iii., 1892. See also Literature of Medulla Oblongata and Pons Varolii. MID-BRAIN, OR MESENCEPHALON, AND REGION OF THIRD VENTRICLE, OR THALAMENCEPHALON (INTER-BRAIN). The parts of the brain next to be described are entirely covered by the cerebral hemispheres. They comprise the crura cerebri and corpora quadrigemina, the optic thalami with the middle commissure, and the pineal body, in addition to the following structures which are seen when the brain is removed from the skull and its under-surface or base is examined, viz. :-the posterior perforated space, the corpora albicantia, the tuber cinereum with the infundibulum and pituitary body, the optic tracts and chiasma, and the lamina cinerea. Of these the corpora quadrigemina and crura cerebri are found in connection with the aqueduct of Sylvius, and belong to the mid-brain, while the optic thalami and the other structures above enumerated occur in connection with the third ventricle, and belong to the interbrain. The aqueduct of Sylvius (iter a tertio ad quartum ventriculum) is a narrow passage into which the upper end of the fourth ventricle gradually narrows, and which in front expands abruptly into the third ventricle. It is rather more than Fig. 72.-SECTIONS THROUGH THE ORIGIN OF THE FOURTH NERVE (Stilling). A, transverse section at the place of emergence of the nerve-fibres. B, oblique section carried along the course of the bundles from the nucleus of origin to the place of emergence. Aq, Sylvian aqueduct, with its surrounding grey matter; IV, the nerve-bundles emerging; IV', decussation of the nerves of the two sides; IV", a round bundle passing downwards by the side of the aqueduct to emerge a little lower down; n. IV, nucleus of the fourth nerve. l, fillet; s. c. p., superior cerebellar peduncle; d. V., descending root of the fifth nerve; pl, posterior longitudinal bundle; r, raphe. half an inch long. In shape it varies in different parts, being T-shaped in section. below (near the fourth ventricle), triangular above (near the third), and in the intermediate part of an elongated oval form, but somewhat shield-shaped in the region of the superior corpora quadrigemina. It is lined by ciliated columnar epithelium, outside which is a thick layer of grey matter, continuous with that of the fourth ventricle. Outside this central grey matter of the aqueduct, the lateral and ventral parts (basal part) of the mesencephalon are composed of the thick masses of the cerebral peduncles (crura cerebri), whilst the dorsum is formed by the lamina quadrigemina, so called from bearing the four mamillated tubercles known as the corpora quadrigemina. The epithelium which lines the Sylvian aqueduct is ciliated (as elsewhere in the ventricles of the brain) and the attached ends of the cells extend as radiating ependymal fibres through the thickness of the mid-brain to reach the surface-at least, this can be seen to be so in the embryo and in small vertebrates, and is probably also true for all. But some of the ependymal fibres are attached to neuroglia cells which occur at various levels in the course of the fibres; they have probably been formed by the detachment of some of the epithelium cells. The fibres which extend from them often branch dichotomously besides possessing many small lateral offsets. The Sylvian aqueduct expands suddenly immediately after passing beneath the posterior commissure into a comparatively large, laterally compressed cavity, termed the third ventricle (fig. 73). This, which is deeper in front than behind, passes at its anterior and lower extremity to a conical termination which lies over the Fig. 73.-VIEW FROM ABOVE OF THE THIRD VENTRICLE AND A PART OF THE LATERAL VENTRICLES (Henle).. The brain has been sliced horizontally immediately below the corpus callosum, and the fornix and velum interpositum have been removed. Tho, thalamus opticus; Ts, its anterior tubercle; P, pulvinar; Com, middle commissure stretching between the two optic thalami across the middle of the third ventricle; Cf, columns of the fornix; Cn, pineal gland projecting downwards and backwards between the superior corpora quadrigemina; St, stria terminalis; Cs, nucleus caudatus of the corpus striatum; Vsl, ventricle of the septum lucidum; Ccl2, section of the genu of the corpus callosum; Pen, commencement of the pineal stria or peduncle, Tfo; Cop, posterior commissure. optic commissure (optic recess). over the pineal stalk; this is termed the suprapineal recess (fig. 90). The ventricle is bounded laterally by the optic thalami (fig. 73, Tho), which come almost in contact with one another in the median plane; and a little in advance of the middle of the ventricle, are actually united by a connecting band of grey matter of variable extent, termed the middle or soft commissure (fig. 73, Com. ; fig. 83, m.c.). This is sometimes double and occasionally wanting it is liable to be torn across in removing the brain. The lateral walls of the cavity are slightly convex, and each is marked towards the anterior end by a white curved band, with its convexity forwards, which becomes more prominent as it passes upwards towards the roof. These bands are named the anterior pillars or columns of the fornix (fig. 73, Cf). Immediately behind the most prominent part of each of these, between it and the anterior part of the thalamus, is an aperture (foramen of Monro) leading into the ventricle of the hemisphere (lateral ventricle.) VOL. III. H |