64 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



aquatic vertebrates it divides into two main trunks, a ramus 

 lateralis (possibly equivalent to the r. dorsalis of a spinal 

 nerve), which is lacking 'in the terrestrial forms, and a ramus 

 intestinalis. The lateralis branch runs the length of the body, 

 either close beneath the skin, or deeper in the muscles near the 

 vertebral column. It is purely sensory, and is distributed to 

 the lateral line organs .of the trunk ; and the absence of these 

 structures in the amniote vertebrates explains the disappearance 

 of the nerve. The ramus intestinalis is the pneumogastric 

 nerve of human anatomy. It is largely motor (or better, in- 

 hibitory) in its functions. It is distributed to pharynx, stomach 

 (air-bladder of fishes), and the respiratory apparatus, gills and 

 lungs. Of the branches to the gills there are as many as there 

 are gill clefts behind the one supplied by the ninth nerve. Each 

 branch divides above the gill cleft into pre- and post-trematic 

 branches. 



The accessory of Willis is apparently a spinal nerve which 

 in the amniotes enters into close association with the vagus. 

 Its distribution is chiefly to the muscles connected with the 

 neck and shoulder girdle, e.g., sternocleidomastoid and trapezius. 



The hypoglossal nerve is, in the adult vertebrate, purely 

 motor, its branches being distributed to the muscles of the 

 tongue and to some of those of the hyoid region. It is only in 

 the amniotes that this can be considered as a cranial nerve ; 

 in the ichthyopsida it does not enter the skull. It is interest- 

 ing to find that in the larval stages of some forms this nerve 

 has a dorsal ganglionated root, while in certain species two such 

 roots have been found, a fact which tends to show that the nerve 

 is really compound. 



Within recent years it has been recognized that the compo- 

 nents of the spinal and cranial nerves were more numerous than 

 is implied by the account given on pages 46 and 59. In the 

 spinal nerve it is clear that a distinction must be made between 

 the nerves of the body (somatic nerves) and those of the viscera 

 (visceral nerves). Each of these is made up of sensory and 

 motor parts, so that four components are to be recognized: (i), 

 somatic sensory (general cutaneous) ; (2), somatic motor ; (3), 

 visceral sensory ; and (4), visceral motor. The ganglion cells 



