880 



THE NER VE SYSTEM 



These defined nests of nerve elements, from their relations to the cranial nerve 

 roots, are called the cranial nerve nidi or nuclei. Their analogy to the origins 

 of the spinal nerves extends to the shape and character of their cell elements 

 and their differentiation into (a) nuclei of origin and (6) nuclei of termination or 

 recipient nuclei. 



The nuclei of origin or motor nuclei are cell clusters from which arise the axones 

 of efferent nerves or the efferent components of the mixed nerves. Some of these 

 nuclei are in line with the basal portion of the ventral gray horn in the cord below 



and are termed, owing to their situ- 

 ation near the mesal plane, the 

 medial nuclei of origin. Other nuclei 

 are isolated cell columns in the line 

 of the caput cornu ventrale detached 

 by the ducussation of the pyramids, 

 termed, from their position in the 

 tegmental substance, the lateral nuclei 

 of origin. The different nuclei of 

 origin of the efferent cranial nerves 

 are under the dominance of the cere- 

 bral cortex by way of the cortico- 

 tegmental (or corticobulbar) path 

 usually included in the pyramidal 

 tract. 



The nuclei of termination or sen- 

 sor cranial nerve nidi are likewise 

 repetitions in structure of the dorsal 

 horn of the spinal gray, but with less 

 regularity and definiteness of posi- 

 tion. Thus, while the gelatinosa 

 Rolandi of the cord is continuous 

 with the nucleus of the spinal root of 

 the trigeminal nerve, other recipient 

 or afferent nuclei are more or less 

 isolated in the tegmental substance, 

 while the two (lateral and ventral) 

 nuclei of the cochlear nerve actually 

 lie on the surface of the brain stem. 

 The afferent impulses carried in 

 by the sensor cranial nerves excite 

 impulses in the neurones of the nuclei 

 of termination; their axones enter 

 the tegmental substance as arcuate 

 fibres, cross the mesal plane to join 

 the lemnisci to connect with the thal- 

 amus and posterior quadrigeminal 

 body and via thalamus and posterior 

 quadrigeminal body with the cerebral 



cortex. The location of the various cranial nerve nuclei in the brain stem may be 

 understood by a reference to the diagrams in Figs. 634, 646, 647, and 648. 



Hypoglossal Nerve Nucleus. The nucleus of origin of the hypoglossal nerve is 

 a rod-like cell column close to the mesal plane, extending for about 7 mm. (^ inch) 

 in the caudal portion of the fourth ventricle, while its extraventricular portion 

 extends about 5 mm. ( inch) caudad of the tip of the calamus. Its efferent 

 axones course ventrad between the formatio reticularis alba and grisea, thence 



FIG. 646. The cranial nerve nuclei schematically repre- 

 sented in a supposedly transparent brain stem, dorsal 

 view. Motor nuclei in red; primary terminal nuclei of 

 afferent (sensor) nerves in blue. (Optic and olfactory 

 centres are omitted.) 



