136 ELEMENTS OF COMPARATIVE ZOOLOGY. 



contained in cerebellum and medulla. In the lower verte- 

 brates these five regions are nearly equal in size, but the 

 higher we go in the scale the larger proportionately do the 

 cerebrum and the cerebellum become, until in man the 

 cerebrum weighs about nine tenths of the whole brain. 



Prom the brain are given off, typically, twelve pairs of 

 nerves, which are spoken of both by numbers and by their 

 proper names. The majority of these are unlike the spinal 

 nerves in that they have but a single root, and are corre- 

 spondingly either sensory or motor. Thus the first or olfac- 

 tory nerve, which goes to the nose; the second 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 trigeminal supplies the 

 sense organs of the head and the principal muscles of the 

 jaws. The seventh (facial) goes to the superficial facial 

 muscles, and in the lower vertebrates supplies certain sense 

 organs in the skin, but in man has lost its sensory functions. 

 The ninth (glossopharyngeal) goes to the tongue and 

 pharynx; while the tenth (vagus or pneumogastric) sup- 

 plies the sense organs of the lateral line (p. 137) of the trunk 

 and sends branches to the stomach, lungs, gills, heart, etc. 



Connected with the nervous system are the sense organs, 



* Tbis occurs in no icbthyopsidan vertebrate. 



