THE NERVOUS SYSTEM. 



469 



the trigeminus may be partly due to disappearance or specialization of anterior 

 somatic nerves and also to the growth of the head. 



The organs of the lateral line are innervated by a quite distinct system of 

 ganglionated afferent nerves whose central connections are nearly identical with 

 those of the acoustic (Fig. 405). With the disappearance of the lateral line 

 organs and the specialization of the cochlear part of the ear vesicle, there is a 

 disappearance of the lateral line nerves (comp. Figs. 405 and 406) and a well- 

 marked division of the acoustic nerve into vestibular and cochlear portions, 

 the former innervating the older vestibulo-semicircular canal portion, the latter, 

 the more recent cochlea. Centrally, the vestibular nerve forms also a descend- 

 ing bundle of fibers and has its own more or less specialized terminal nuclei. 

 The latter is also true of the cochlear nerve. 



The afferent portions of the facial, glossopharyngeal and vagus nerves in- 

 nervate the splanchnic receptors of the pharyngeal and branchial surfaces as 

 well as of a large part of the viscera. The facial, glossopharyngeal and vagus 

 also innervate the specialized splanchnic receptors, the gustatory system men- 

 tioned above. This system of taste buds has a very extensive development in 

 certain lower Vertebrates, especially the Bony Fishes. In the latter the 

 system of nerves innervating these structures is naturally much more extensive 

 and its central terminations and nuclei cause important modifications of the 

 medulla. In Mammals the remnants of this system are represented by the 

 taste buds in the mouth, the nerves innervating them being the chorda tympani 

 branch of the facial and the lingual branch of the glossopharyngeal (Fig. 406). 

 The central branches of the ganglia of these three nerves, after entering the 

 brain, form a descending bundle of fibers, the tr actus solitarius (or communis). 



The somatic musculature of the head, as above mentioned, is usually taken 

 to be represented by the eye muscles and, later, the tongue muscles. The 

 tongue is one of the newer structures, rising in importance with the change to 

 a land habitat, and its muscles are probably an invasion from the neck region 

 caudal to the branchial arches (p. 322). The eye muscles are innervated by 

 the III, IV and VI cranial nerves, the tongue muscles by the XII which is a 

 more recent addition to the cranial nerves. All of these nerves are charac- 

 terized by having their neurone bodies located in the most medial (morpholog- 

 ically most ventral) portions of the lateral brain walls, and they all, except the 

 IV, emerge near the mid-ventral line. In these respects they resemble the 

 major or somatic part of the ventral spinal roots. (For illustration see Figs. 

 427, 405 and 406). 



The splanchnic musculature of the jaws and the branchial arches is inner- 

 vated by the efferent portions of the V, VII, IX, X (and XI). The neurone 

 bodies or nuclei of origin of these nerves lie more laterally than those of the III, 

 IV, VI and XII, and their axones also leave the nerve tube more laterally 



