HIGH SCHOOL ZOOLOGY. 33 



the parts of its own metamere, although for puri:>ose of coordi- 

 nation, as e. g., in the various movements of the pectoral fin, 

 the nerves of contiguous metameres communicate with each 

 other in plexuses. The parts above the level of the spinal 

 canal are supplied by the dorsal division of the spinal nerves, 

 those below by the ventral divisions, and finally the contents 

 of the cielom are supplied by special intestinal branches which 

 are provided with ganglia, communicate intimately with each 

 other before supplying the viscera, and constitute the Sympa- 

 thetic system. The arrangement of the cranial nerves is 

 however much more complicated, first, because their metamery 

 is not so evident, and second, because the nerves as they emerge 

 separately from the brain, are not each comi)Osed of a dorsal 

 and ventral root, but some seem only to be ventral, others to 

 be composed of several dorsal and venti'al roots. Two of them, 

 the 1st and 2nd paii'S, olfactory and optic, go to the nose and 

 eyes respectively, the 3rd, 4th and 6th, are motor nerves which 

 control the muscles of the eyeball, the 5th and 7th supply the 

 greater part of the head with sensory and motor nerves, the 

 8th is distributed only to the internal ear, while the 9th chiefly 

 ends in the 1st gill arch, and the 10th is distributed to the re- 

 maining gills, but does not confine itself to the head, and sends 

 branches to the heart, air-bladder and stomach. A separate 

 branch of this widely-distiibuted (Vagus) nerve supplies the 

 sense-organs of the lateral line. The fifth nerve is also not con- 

 fined to the head, but communicates with the dorsal branches 

 of the spinal nerves by a long branch which pierces the back 

 of the skull on either side of the supra-occipital spine. 



40. We tuust now turn our attention to the endin"s of these 

 nerves, especially to those of the afierent nerves, which tx-ans- 

 mit impulses to the brain and spinal cord from the sense-organs. 

 Certain of the latter have been already referred to (§ 9), there 

 remain for discussion the higher sense organs, or the nose, eye 

 and ear. 



