THE NERVOUS SYSTEM 



457 



The lingual branch of the Glosso-pharyngeus, which appears 

 first in the Dipnoi, becomes in the Amniota, and especially in 

 the mammals, a large and important nerve, rnd specializes as 

 the nerve of taste (gustatory). The remaining branch is 

 motor and is distributed to those muscles which are derived 

 from those of the first branchial arch. In the same way motor 

 branches of the Vagus supply the muscles of the larynx and 

 trachea, and the walls of the pharynx. 



The ramus lateralis disappears with the advent of terrestrial 

 life, but, on the other hand, the increase in size and complexity 

 of heart, lungs, oesophagus, and stomach so increase the im- 

 portance, of ramus intestinalis that it alone comes to be con- 

 sidered the main nerve (hence the name " Pneumo-gastric "), 

 of which the other elements are considered branches. 



The circumstance which leads in Sauropsida to the partial, 

 and in mammals to the complete, separation of the Accessorius 

 is clearly found in the greater development and higher degree 

 of independence of the parts to which it is supplied, the trape- 

 zius and sterno-cleido-mastoid muscles, a development due 

 in its turn to the increased importance of the neck. This 

 nerve is still a part of the Vagus in the human embryo and 

 shows the steps of its gradual emancipation during develop- 

 ment (Fig. 125). 



V. HYPOGLOSSUS. 



The Hypoglossal nerve, which appears in the higher verte- 

 brates as the last of the cranial nerves, is plainly one or more 

 adopted spinal nerves, found still in their original office in 

 fishes and amphibians. It is mainly a motor nerve and arises 

 from several roots which belong in the ventral series; this 

 is rendered more certain by the appearance, usually transi- 

 tory and embryonal, of corresponding dorsal roots, equipped 

 with ganglia, which thus complete the elements necessary for 

 genuine spinal nerves. In Plate VII these latter are indicated 

 by dotted lines; and the two spinal nerves which are shown 

 in Plate VI may be considered to represent the potential 

 hypoglossal still in an indifferent condition. Some of the 

 spino-occipital elements may also enter into the formation of 



