NERVUS TRIGEMINUS.
Here a motor part and a sensory part are to be distinguished (Figs. 163 and 164). t. Motor Portion. The central neurone takes origin in the cerebral cortex of the lower third of the.central convolutions, passes with the pyramidal tracts downward and ends in the chief motor nucleus, within the dorso-lateral part of the tegmentum of the pons. The peripheral neurone arises within this motor nucleus, the motor root of the nerve receiving also fibres from the nucleus of the opposite side. The fibres emerge from the pons as the fiortio minor nervi trigemini and pass to the muscles. A small part of the motor root arises from small cells, which lie lateral to the Sylvian aqueduct within the region of the corpora quadrigemina and constitute the nucleus radicis descendentis nervi frigemini. This group of cells joins caudally the cell-area of the locus caerulus. The fibres arising from these cells, after giving off collaterals to the chief motor nucleus, pass outward with the other peripherally directed processes of the motor neurones.
2. Sensory Portion. The origin of the sensory part of the trifacial nerve lies within the ganglion Gassed. The axones of the unipolar ganglion cells of this ganglion divide into two branches. One of these extends peripherally as the peripheral nerve, the other passes centrally, enters the pons as the fiortio major nervi trigemini, and runs to the sensory of the trigeminus, close to the motor nucleus. Here each fibre divides into an ascending and a descending branch. The ascending branch ends
within the sensory nucleus within the pontile tegmentum. The descending branch ends, after giving off numerous collaterals, in a nucleus that is nothing more than the caudal prolongation of the sensory nucleus. The descending branches form collectively the tractus spinalis nervi trigemini; the gray substance in which this path ends, constitutes the nucleus tractus nervi trigemini. The descending tract, as well as the nucleus, can be followed downward as far as the cervical cord, the nucleus being identical with the substantia gelatinosa Rolandi capping the posterior horn. From the sensory end-nucleus arises the II neurone. The fibres pass towards the mid-line, giving off collaterals to the nucleus of the facial nerve, cross to the fillet-tract of the opposite side, there turn upward and run forward (partly within the medial fillet and partly as a more laterally placed special ascending bundle), and later enter the thalamus with the medial fillet. Finally, a III neurone succeeds the second one, thus linking the thalamus with the cortical sensory area. Still to be mentioned are sensory fibres, which pass direct to the cerebellum as constituents of the direct sensory cerebellar tract; further, fibres which pass from the sensory end-nucleus to the cerebellum as constituents of the tractus nucleo-cerebellaris.