FIBRE-TRACTS OF THE CEREBELLUM.
All cortical regions of the cerebellum are linked together by means of arched fibres, the fibrae arciformes. Such association systems unite neighboring folia or lobules of the cerebellum. The cortex, moreover, sends centrifugal fibres to the nuclei—to the nucleus dentatus and the nucleus fastigii, as well as to Deiters' nucleus.
The chief connections of the cerebellum are: z. Tractus composed of fibres which arise in the pontile nuclei and proceed to the cerebellar hemisphere of the opposite side. These tracts form the middle cerebellar peduncle. Since the ponto-cerebellar tract continues the tradus corticu ad pontem, connecting the cere bral cortex with the pontile nucleus, impulses are carried from the cerebrum to the cete bellum. Relations between the cerebral cortex and the cere bellum may be further established by way of the thalamus and the inferior olive.
Some fibres pass in the opposite direction, from the cere bellum through the middle cere bellar peduncle to the pons and thence, as the fibrae rectae dorsally within the raphe of the pons to the centro-lateral nucleus redicularis tegmenti pontis, thus constituting the tractus fiontis.
2. Tractus composed of fibres which arise in the nucleus dentatus and partly in the nucleus fastigii or roof-nucleus of the cerebellum, pass for ward, decussate in the quadrigeminal region and end within the nucleus ruber or the thalamus. They constitute the superior cerebellar peduncle or brachium conjunctiva, the crossing being known as the decussation of the superior peduncle. These fibre bundles, chiefly from the nucleus dentatus cerebelli, give off descending collateral branches, which may be followed as a special bundle as far as the pons and the medulla oblongata, where they probably end in motor nuclei. In addition to this robust cerebellofugal tract, other efferent bundles pass caudalward from the roof nucleus, of the same and the opposite side, into the tegmental area of the medulla oblongata, where they end around the cells of the substantia reticularis. These fibres,
however, emerge from the cerebellum by way of the inferior cerebellar peduncle or restiform body and are specially designated as the tractus while those in the superior peduncle are called the tractus mesencephali.
By means of the middle cerebellar peduncles, therefore, especially impulses from the cerebrum are conveyed to the cerebellum. The superior cerebellar peduncle, on the contrary, carries impulses by way of the red nucleus and the thalamus from the cerebellum to the cerebrum. In addition, however, by way of the tractus cerebello tegmentalis mesencephali (cerebellum to red nucleus), as well as by the tractus cerebello tegmentalis pontis et bulbi, the possibility exists, that impulses from the cerebellum may be finally transferred to the motor nuclei of the cerebral nerves and to the gray substance of the spinal cord. Since the tractus rubro-reticularis passes from the red nucleus to the cells of the formatio reticularis of the pons and of the medulla oblongata and the tractus rubro-spinalis passes to the spinal cord, while other fibres spring from the cells of the formatio reticularis, around which also the tractus cere bello-tegmentalis pontis et bulbi end, certain fibre-strands reach far down into the medulla and the spinal cord.
3. Constituents of the inferior cerebellar peduncle, fibres which come from the spinal cord and the medulla oblongata and pass to the cerebellum by way of the peduncle or restiform body. The constitution and destination of the latter will be further considered in connection with the fibre-tracts of the medulla oblongata (page 17o).
The chief connections of the pons and of the cerebellum are represented in Figs. and 147. It is to be noted, further, that numerous tracts, descending as well as ascending, traverse the pons ( Figs. 142 and 147).