300 SYSTEMATIC ZOOLOGY. 



or optic nerve, to the eye; the eighth or auditory nerve, 

 distributed to the ear, are purely sensory. On the other 

 hand, the third, fourth, and sixth (oculomotor, trochlearis, 

 and abducens) nerves go to the muscles of the eye; the 

 eleventh * (accessorius) goes to the muscles of the shoulder- 

 girdle; and the twelfth (hypoglossal) goes to the muscles of 

 the tongue. These nerves are purely motor, but it must 

 be remembered that the twelfth in the young of a few 

 forms has a dorsal ganglionated root. The remaining 

 nerves are like the spinal nerves in so far as they have 

 both sensory and motor functions. The fifth or trigem- 

 inal supplies the sense-organs of the head and the princi- 

 pal muscles of the jaws. The seventh (facial) goes to 

 the superficial facial muscles, and in the lower vertebrates 

 supplies certain sense-organs (lateral-line organs) in the 

 skin, but in man has lost its sensory functions. The ninth 

 (glossopharyngeal} goes to the tongue and pharnyx; while 

 the tenth (vagus or pneumogastric) supplies the sense- 

 organs of the gill-slits and of the lateral line (below) of 

 the trunk and sends branches to the stomach, lungs, gills, 

 heart, etc. It will thus be seen that the vagus nerve cf 

 the lower vertebrates is more than the pneumogastric of 

 the terrestrial forms. 



Connected with the nervous system are the sense-organs. 

 The skin contains small touch or tactile organs connected 

 with afferent nerves, and these are for the recognition of 

 pressure and temperature. Possibly allied to these are 

 the organs of the lateral line, which are found only in the 

 aquatic Ichthyopsida. These organs are sometimes free 

 on the surface, sometimes in pits, while not infrequently 

 the pits are connected by canals running beneath the 

 surface, with openings to the exterior here and there. 

 * This occurs in no ichthyopsidan vertebrate. 



