862 NEUROLOGY 



cortex are conducted by the pyramidal tract fibers (corticopontine fibers). These 

 fibers probably terminate in relation with association neurons which control the 

 coordinated action of all the eye muscles. This association and coordination 

 mechanism is interposed between the terminals and collaterals of the voluntary 

 fibers and the neurons within the nuclei of origin of the motor fibers to the eye 

 muscles. The fibers of the posterior longitudinal bundle are supposed to play an 

 important role in the coordination of the movements of the eyeball. Whether it is 

 concerned only with coordinations between the vestibular apparatus and the eye 

 or with more extensive coordinations is unknown. Many fibers of the posterior 

 longitudinal bundle have their origin in the terminal nuclei of the vestibular nerve 

 and from the posterior longitudinal bundle many collaterals and terminals are 

 given off to the abducent nucleus as well as to the trochlear and oculomotor nuclei. 

 The abducens nucleus probably receives collaterals and terminals from the ventral 

 longitudinal bundle (tectospinal fasciculus) ; fibers which have their origin in the 

 superior colliculus, the primary visual center, and are concerned with visual reflexes. 

 Others probably come from the reflex auditory center in the inferior colliculus and 

 from other sensory nuclei of the brain-stem. 



The Trigeminal Nerve (V cranial) contains somatic motor and somatic sensory 

 fibers. The motor fibers arise in the motor nucleus of the trigeminal and pass 

 ventro-laterally through the pons to supply the muscles of mastication. The sensory 

 fibers arise from the unipolar cells of the semilunar ganglion; the peripheral branches 

 of the T-shaped fibers are distributed to the face and anterior two-thirds of the 

 head; the central fibers pass into the pons with the motor root and bifurcate into 

 ascending and descending branches which terminate in the sensory nuclei of the 

 trigeminal. 



The motor nucleus of the trigeminal is situated in the upper part of the pons 

 beneath the lateral angle of the fourth ventricle. It is serially homologous with 

 the facial nucleus and the nucleus ambiguus (motor nucleus of the vagus and glosso- 

 pharyngeal) which belong to the motor nuclei of the lateral somatic group. The 

 axons arise from large pigmented multipolar cells. The motor nucleus receives 

 reflex collaterals and terminals, (1) from the terminal nucleus of the trigeminal of 

 the same and a few from the opposite side, via the central sensory tract (trigemino- 

 thalamic tract); (2) from the mesencephalic root of the trigeminal; (3) from the 

 posterior longitudinal bundle; (4) and probably from fibers in the formatio reticu- 

 laris. It also receives collaterals and terminals from the opposite pyramidal tract 

 (corticopontine fibers) for voluntary movements. There is probably a connecting 

 or association neuron interposed between these fibers and the motor neurons. 



The terminal sensory nucleus consists of an enlarged upper end, the main sensory 

 nucleus, and a long more slender descending portion which passes down through 

 the pons and medulla to become continuous with the dorsal part of the posterior 

 column of the gray matter especially the substantia gelatinosa of the spinal cord. 

 This descending portion consists mainly of substantia gelatinosa and is called the 

 nucleus of the spinal tract of the trigeminal nerve. 



The main sensory nucleus lies lateral to the motor nucleus beneath the superior 

 peduncle. It receives the short ascending branches of the sensory root. The 

 descending branches which form the tractus spinalis, pass down through the pons 

 and medulla on the lateral side of the nucleus of the tractus spinalis, in wiiich they 

 end by collaterals and terminals, into the spinal cord on the level of the second 

 cervical segment. It decreases rapidly in size as it descends. At first it is located 

 between the emergent part of the facial nerve and the vestibular nerve, then between 

 the nucleus of the facial nerve and the inferior peduncle. Lower down in the upper 

 part of the medulla it lies beneath the inferior peduncle and is broken up into 

 bundles by the olivocerebellar fibers and the roots of the ninth and tenth cranial 

 nerves. Finally it comes to the surface of the medulla under the tubercle of 



