492 



PISCES— FISHES. 



above the vertebral column, is divided by deep dorsal and 

 ventral fissures, and gives off numerous spinal nerves, 

 formed as usual from the union of dorsal (sensory) and 

 ventral (motor) roots. The first sixteen or eighteen nerves 

 form the brachial plexus, which supplies the pectoral fin. 



The sympathetic system consists of a longitudinal gang- 

 lionated cord along each side of the vertebral column. 



»m./ 



Fig. 207. — Side view of chief cranial nerves of Elasmobranchs. 

 — Slightly modified from Cossar Ewart. 



olf., Over olfactory nerve ; ch., over cerebral hemispheres ; cb. } over 

 cerebellum; m.o., over medulla oblongata; ?tz., mouth; mx., 

 maxillary branch of 5 ; mn. 5, mandibular branch of 5 ; 7/m.7, 

 mandibular branch of seventh nerve ; #.1-5, groups of ampullae ; 

 o.s. 5, superficial ophthalmic of 5 ; o.p., ophthalmicus profundus ; 

 o.s.j, superficial ophthalmic of 7 ; N., nostril ; 3, oculomotor ; 

 e.g., ciliary ganglion; 5, trigeminal; i.b., inner buccal; o.b., 

 outer buccal ; jb., buccal of 7 ; /., palatal of 7 ; sp., spiracle ; 

 ch., chorda tympani ; 7. hm, hyomandibular of 7 ; 8, auditory ; 

 E., ear ; 9, glossopharyngeal ; 10, roots of vagus ; /.io, lateral 

 nerve of vagus ; i. 10, intestinal nerve of vagus ; r'-s' , gill-clefts. 



Sense organs. — 



(a) The eyes (see p. 448). The iris has a fringed upper margin. 



(b) The ears (see p. 447). The vestibule is connected with the sur- 



face by a delicate canal — the aqueductus vestibuli — a remnant 

 of the original invagination. A small part of the wall of the 

 auditory capsule is covered only by the skin, forming a kind of 

 tympanum. Within the vestibule are calcareous otolithic par- 

 ticles surrounded by a jelly. 



(c) The nasal sacs are cup-like cavities with plaited walls. 



(d) The sensory tubes are best seen on the ventral surface, where 



they lie just under the skin. At their internal ends lie ampulke, 

 containing sensory cells. At their outer ends there are pores. 

 It is probable that they are organs partly of touch, and partly 

 of ' ' chemical sense. " 



