THE NERVOUS SYSTEM. 471 



along with the incoming afferent fibres. These nerves all exhibit a character- 

 istic segmental arrangement corresponding to that of the gill clefts. The 

 VII, IX, and the various nerves making up the X, divide dorsal to the cor- 

 responding gill clefts into prebranchial and postbranchial branches, also 

 giving off suprabranchial branches. The efferent element, or component, 

 forms a part of each postbranchial branch. These relations are shown clearly 

 in the accompanying diagrams (Figs. 405 and 406). Part of the vagus also 

 innervates the viscera and this nerve is thus divisible into branchial and visceral 

 portions. 



Two peculiarities may be noted in regard to these splanchnic nerves : First, 

 that the afferent portions have ganglia resembling those of the spinal nerves; 

 second, that the branchial efferent portions consist simply of one neurone 

 proceeding all the way from the nerve tube to the muscle innervated, thus 

 resembling the somatic rather than the visceral nerves of the trunk. As al- 

 ready noted (p. 466), these nerves regulate activities somatic in character but 

 involving splanchnic structures. It is thus seen that the dominating factor is 

 functional rather than morphological present functional necessities modify 

 those of the past. 



With the change from a water to a land habitat and the accompanying 

 disappearance of gills and appearance of lungs, we have various suppressions 

 and modifications of the branchial musculature (Fig. 406). There are two 

 striking specializations of the branchial musculature. One is the origin of 

 the facial (mimetic) musculature in the highest Vertebrates. This is derived 

 from the muscles of the hyoid arch, innervated naturally by extensions of the 

 facial nerve. The other is a specialization of muscles, probably of the caudal 

 branchial arches, into cervico-cranial muscles (head-movement), innervated by 

 what may be considered a caudal extension of the vagus nerve, namely, the 

 spinal accessory (p. 503). The splanchnic laryngeal musculature and its 

 nerves show a certain degree of specialization (sound-production) in higher 

 forms. The efferent V is naturally a large constant nerve, in correlation with 

 the uniformly developed jaw musculature in all jaw-bearing (gnathostome) 

 Vertebrates (Figs. 405 and 406). These various changes in peripheral 

 structures are thus due either to environmental influences or to developments 

 within the central nervous system (p. 457). One of the most important en- 

 vironmental influences is the change from a water to a land habitat. The 

 influence of the central nervous system is shown in the further development 

 and specialization of a number of peripheral structures as motor "instru- 

 ments" of suprasegmental mechanisms. 



The effects, then, of these peripheral arrangements upon arrangements 

 within the neural tube are: (i) The formation of separate tracts and terminal 

 nuclei for (a) the unspecialized somatic afferent V nerve (spinal V and posterior 



