B. REFLEX TRACTS.
The simplest reflex path is established by the reflex collaterals. In this case only two neurones share the entire path, the transference from the centripetal to the centrifugal neurone being accomplished by means of the collaterals given off directly from the centripetal or afferent neurone.
The release of the reflex may be in duced, however, by intercalated neurones. Thus, between the centripetal and the cen trifugal neurone a third neurone may inter vene, thereby making possible the transfer ence of the impulse conveyed by a single centripetal neurone to several centrifugal ones. Such intercalated neurones, for ex ample, are the association-cells of the spinal cord, which distribute, by means of their axones and collaterals, impulses to many cells within the cord-segments of higher and lower levels. To this category belongs, fur ther, the posterior longitudinal bundle. Im pulses carried to Deiters' nucleus by the vestibular nerve may be distributed to the nuclei of the eye-muscles and to the motor cells of the cord by means of fibres, which proceed from Deiters' nucleus and run within the posterior longitudinal bundle. In consequence of the introduction of sev eral neurones between the centripetal and the centrifugal conduction, the entire reflex mechanism may become very complex.
The cerebellum, with its afferent and efferent paths, calls for special consideration. The cerebellum is the centre for the reflex and unconscious maintenance of equilibrium, during rest as well as during changes in the position of the centre of gravity. The centripetal paths lie especially within the nervus vestibuli and within the ascending fibre systems from the spinal cord and from the medulla oblongata. These ascending paths from the cord are the tractus spino-cerebellaris dorsalis and ventralis ; from the medulla oblongata the fibres arising within the nucleus gracilis and cuneatus. An indirect con duction from the spinal cord is perhaps effected by the tractus spino-olivaris or Helweg's triangular tract, which ends within the inferior olivary nucleus, and thence by the tractus olivo-cerebellaris to the cerebellum, by way of the restiform body. The direct and the
indirect sensory cerebellar tract, as well as the tracts from the quadrigeminal region, are also included among the centripetal paths. By means of the cerebellofugal tracts, impulses may be carried from the cerebellum to other paths and by means of the latter, in turn, be transferred to motor paths. The chief cerebellofugal tracts proceed from Deiters' nucleus and from the nucleus dentatus. From Deiters' nucleus arise the tractus vestibulo spinalis and the posterior longitudinal bundle, the last-named system coming into relation with the spinal cord and with the nuclei of the nerves supplying the ocular muscles— that is, binding together the centres concerned in maintaining equilibrium and relations to space. From the nucleus dentatus arises the superior cerebellar peduncle, whose fibres end within the nucleus ruber, whence the tractus rubro-spinalis passes to the spinal cord.
It is to be noted, that the relations of the cerebellar hemispheres with the spinal cord are homolateral or of the same side. Additional cerebellofugal paths are those which, as the tractus tegmentalis pontis et bulbi, run within the tegmental region of the pons and medulla oblongata, whereby transference to motor nuclei may in turn be effected.
If the maintenance of equilibrium be adjusted to a voluntary movement, the cere bellum is also directly stimulated from the cerebral cortex. The paths for such impulses are the frontal and the temporo-occipital cortico-pontile tracts, which end in the pontile nuclei, whence the conduction to the cerebellum is by the middle cerebellar peduncle. In addition, the pontile nuclei are under the influence of the pyramidal tract, from which, within the pons, collaterals are given off to the nuclei. By means of the superior cerebellar peduncle (cerebellum-nucleus ruber—thalamus—cortex), the cerebellum sends impulses to the cerebral cortex and thereby influences conscious innervation (Figs. 145 and 147).