i8o 



THE NERVOUS SYSTEM 



to their exit from the brain (Fig. 128). This nucleus, which supplies the muscles that sur- 

 round the mouth, receives axons from the primary taste center in the medulla oblongata 

 (the nucleus of the tractus solitarius) which is located at a more caudal level. Accordingly, 

 the facial nucleus migrates caudally toward that center. It also receives fibers from the 

 nucleus of the spinal tract of the trigeminal nerve and migrates ventrolaterally toward it. 

 Thus is explained the adult position of the nucleus of the facial nerve, not far from the 

 spinal tract of the trigeminal nerve and near the rostral end of the nucleus of the tractus 

 solitarius. In the same way the curved course of the facial nerve within the pons may be 

 explained. These examples are perhaps sufficient to illustrate the general principle of neuro- 

 biotaxis. 



Nuclei of Origin and Terminal Nuclei. The efferent nuclei, which we have 

 examined, all have this in common, that the axons, which take origin from their 

 constituent cells, leave the brain through the efferent roots of the cranial nerves. 

 Hence they may all be included under the term nuclei of origin. On the other 

 hand, the afferent fibers of the cerebrospinal nerves have their cells of origin located 



Genu internum n. facialis 







Sulcus 



Sulcus 



Sulcus 



Fig. 128. Diagram illustrating three stages in the development of the genu of the facial 

 nerve, the youngest, A, corresponding to the 10 mm. embryo, and the oldest, C, the newborn 

 child. The relative position of the nucleus of the n. abducens is represented in outline. Sulcus, 

 Sulcus medianus fossae rhomboideae. (Streeter, in Keibel and Mall's Embryology.) 



outside the central nervous system and, with the exception of the first two cranial 

 nerves, in the cerebrospinal ganglia. These fibers enter the central nervous 

 system and end by entering into synaptic relations with sensory neurons of the 

 second order located in terminal nuclei. These are classified according to the 

 function of the fibers which end in them as visceral afferent and somatic afferent 

 nuclei. 



THE VISCERAL AFFERENT COLUMN 



All of the visceral afferent fibers of the cranial nerves, except those of the first 

 pair, are contained in the facial, glossopharyngeal, and vagus nerves. These 

 include: (1) the fibers from the taste buds, which since they mediate the special 

 sense of taste, may be called special visceral afferent fibers; as well as (2) others 

 from the posterior part of the tongue, and from the pharynx, larynx, trachea, 

 esophagus, and thoracic and abdominal viscera, which are known as general 



