144 INTRODUCTION TO NEUROLOGY 



fibers), and the muscles related with the hyoid bone and the superficial 

 facial muscles or muscles of facial expression, these two groups of muscles 

 belonging to the series of special visceral muscles (p. 94). The glosso- 

 pharyngeal nerve (IX pair) supplies fibers to the taste-buds on the posterior 

 third of the tongue (special visceral sensory), also general sensibility to this 

 region, motor fibers for the stylopharyngeus muscle (special visceral motor), 

 and excito-glandular fibers for the parotid salivary gland (general visceral 

 efferent). It also cooperates with the vagus nerve in innervating the skin 

 about the external auditory canal (by the auricular branch of the vagus). 

 The vagus nerve (X pair) is very complex. In addition to the general so- 

 matic sensory fibers of the auricular branch, which have just been men- 

 tioned, it contains general visceral sensory fibers from the pharynx, lungs, 

 stomach, and other abdominal viscera, and visceral efferent fibers of several 

 sorts to the pharynx, esophagus, stomach, intestines, lungs, heart, and 

 arteries. The peripheral and central courses of most of these functional 

 systems have been accurately determined, but are far too complex for sum- 

 mary here. The accessory nerve (XI pair) contains two parts: (1) the 

 bulbar part, which should be regarded as nothing other than detached 

 filaments of the vagus, for all of these fibers peripherally join vagus branches, 

 (2) the spinal part, which arises by numerous rootlets from the upper levels 

 of the spinal cord and participates in the innervation of two of the muscles 

 of the shoulder (the trapezius and sternocleidomastoid muscles). The hu- 

 man hypoglossus nerve (XII pair) is a modified derivative of the first spinal 

 nerve of lower vertebrates. It has lost its sensory fibers and innervates 

 a special part of the tongue musculature. 



All of the nerves of the preceding list except the I, II, III, and 

 IV pairs connect with the medulla oblongata. In the dogfish 

 we have seen that this region of the brain presents special emi- 

 nences which form respectively the terminal nuclei of the 

 acoustic (and lateral line), cutaneous, and visceral (including 

 gustatory) sensory systems (see p. 112 and Figs. 42^14). The 

 primary motor centers lie ventrally of these sensory areas. 



The cranial nerves are usually described in our text-books 

 as if they were segmental units like the spinal nerves (see p. 

 125). This was, in fact, the primitive condition; but in all 

 vertebrate animals this segmental pattern has been greatly 

 modified in such a way as to facilitate the discharge into the 

 brain of all sensory fibers of like physiological type into a single 

 center. These physiological systems are, accordingly, the most 

 useful units of structure in the cranial nerves. Each cranial 

 nerve may contain several of these functional systems, and no 

 two pairs of cranial nerves have the same composition. The 

 components of the cranial nerves, like those of the spinal nerves 

 (p. 126), are named in accordance with the same physiological 

 criteria as their end-organs (see pp. 79-94). 



