The constitution of the corpus restiforme, or the inferior cerebellar peduncle, may now be considered. Although the tracts ascending to the cerebellum from the spinal cord and the medulla are the principal factors in its make-up, additional paths of especial importance are contributed to the restiform body by the fibre-bundles from the vestibular nerve and its end-nucleus.
The restiform body consists of two chief parts, a lateral and a medial division. The lateral division is formed by the following fibre-bundles: a. The tractus ciorsah:s or direct cerebellar tract. Although the tractus spino-cerebellaris ventralis or Gowers' bundle likewise passes to the cerebellum, it reaches the latter, not by way of the restiform body, but, farther above, in conjunction with the superior cerebellar peduncle (Figs. 157 and 178).
b. The fibres from the nucleus gracilis and cuneatus of the same and of the opposite side: fibrae arcuatae externae dorsales el ventrales, as well as direct fibres from the posterior column.
c. A few fibres from the nuclei arcuati or pyramidal nuclei.
d. Fibres from the lateral column nuclei.
e. Fibres from the inferior olivary nucleus. .
The last fibres—tractus olivo-cerebellaris—contribute the chief bulk of the lateral division of the peduncle. They arise in largest part from the contralateral olive, a few fibres coming also from the olive of the same side, and end, as do the other fibres of the lateral division, within the cortex of the worm.
The medial division consists of two chief varieties of fibres: a. One set of fibres is the sensory root-fibres of certain cerebral nerves, as the trigeminus and the vestibular, which pass direct to the cerebellum and constitute the direct sensory cerebellar tract of Edinger.
b. The other fibres connect the nuclei of the sensory cerebral nerves with the cere bellum; among these connections, those of the vestibular nucleus with the cerebellum deserve special notice.
The termination of the fibres of both sets is, in largest part, within the nucleus tegmenti of the cerebellum. Fibres pass also in the opposite direction from the nucleus tegmenti or roof nucleus to the end-nuclei of the sensory nerves, Deiters' and Bechterew's nuclei especially receiving such fibres. These bundles, that bring the nuclei of the sensory cerebral nerves into relation with the cerebellum, constitute the tractus The latter forms an indirect sensory cerebellar tract, in contrast to the above-mentioned direct one. Fibres from the cerebellum also pass caudally to the medulla oblongata by way of the restiform body.
The tractus or is a descending bundle, which proceeds especially from the nucleus tecti of the opposite side and, perhaps, also from the nucleus dentatus. The bundle is known also as the tractus uncinatus (Russel-Thomas). The fibres pass above the superior cerebellar peduncle and, in their farther course, reach the medial division of the restiform body. Their termination is partly within Deiters'
nucleus and partly, farther caudalward, within certain nuclei of the medulla Oblongata, along with collaterals to the motor nuclei of cerebral nerves—V, VII and X. Atten tion has been called to these paths, as the tractus bulbi, when considering the chief connections of the cerebellum. Such cerebellofugal tracts proceed from the restiform body, with the fibrae arcuatae externae ventrales, to the olive and the pyramid and ascend within the raphe to the •ormatio reticularis of the medulla oblongata.
The inferior olivary nucleus, as we have seen, gives off a robust fibre-bundle, which passes to the corpus restiforme of the opposite side and, thence, to the cerebellum. A small number of fibres, on the other hand, arise within the cerebellar cortex and descend to the opposite olive. The olivary nucleus possesses still further connections. Thus, from the ascending tractus spino-cerebellares collaterals are sent to the inferior olivary nucleus, while other fibres are received from the tractus spino-olivaris or Heiweg's tri angular tract, as well as from the tractus thalamo-olivaris. By means of the last-mentioned path, impulses from the thalamus, and also from the cerebral cortex by way of the thalamus, are carried to the inferior olive and, by way of the olivo-cerebellar tract, to the cerebellum (Fig. 145).
The medial fillet or interolivary stratum appears in cross-sections of the medulla as a field, that lies between the two olives at the sides of the raphe (see Part III). Dorsally, as the apex of this field, the posterior longitudinal bundle or the fasciculus lemgitudinalis medialis is seen as a small bundle of longitudinally coursing fibres. It will be considered more fully in connection with the vestibular nerve (page 182). Lateral to the interolivary stratum and dorsal to the olive, a field spreads out which, in addition to numerous scattered nerve-cells, contains longitudinally coursing nerve-fibres. This area, the upward continuation of the formatio reticularis of the spinal cord, is known as the association field of the medulla oblongata. The formatio reticularis extends far up into the mid-brain and contains numerous connecting paths, of longer or shorter course, by which manifold relations are established between certain nerve-nuclei. It is probable that within this formatio reticularis run those association fibres which unite the nuclei of the vagus, facial and phrenic nerves for coordinated activity during respiration. Repeated mention has been made of tracts, coming from other parts of the brain, which have their ending within this formatio reticularis. The reader should refer to the section on the Reflex Tracts (page 192), as well as to the microscopical illustrations in Part III.