FUNCTIONAL DIVISIONS OF NERVOUS SYSTEM 367 



muscles so innervated are never striped, voluntary nor derived 

 from the segmented myotomes. 



The ramus communicans serves not only for the passage of 

 the visceral efferent fibres, but also for the visceral afferent 

 fibres, which then continue to the spinal cord through the 

 dorsal root. 



In the region of the head, a slight complication is introduced 

 owing to the development of special sense-organs, and to the 

 fact that the anterior region of the alimentary canal is modified 

 in connexion with the jaws and gill-arches. There is further 

 the fact that the dorsal and ventral nerve-roots of the cranial 

 segments remain separated and do not join to form a mixed 

 nerve. 



The various nerve-components in the head can conveniently 

 be studied in the dogfish. Leaving aside for the moment the 

 very specialised visual and olfactory organs, the somatic 

 afferent system is divided into two owing to the development 

 of the lateral-line system. 



There is, therefore, a general somatic afferent system which 

 receives impulses from simple sense-organs in the skin corre- 

 sponding to those in the region of the trunk and spinal nerves. 

 This component is present in the trigeminal, glossopharyngeal, 

 and vagus, and their fibres end in the dorsal portion of the 

 medulla oblongata in a region which may be called the " skin- 

 brain." 



The special somatic afferent system is concerned with the 

 lateral-line organs and the special member of these which is 

 the ear. This component is present in the facial (superficial 

 ophthalmic, buccal and hyomandibular branches), auditory, 

 glossopharyngeal and vagus, and its centre is also in the dorsal 

 part of the medulla oblongata. ^3o great is the number of 

 fibres which end in this way, that the neurons in the medulla 

 with which the afferent fibres make connexion are also multi- 

 plied. The result is that this region, which may be called the 

 " ear-brain," bulges out, forming the tuberculum acusticum. 

 The special somatic afferent system is also called the lateralis 

 system, and arises in relation to the dorso-lateral placodes of 

 the 7th, 9th, and 10th cranial nerves (see p. 194). 



