528 TEXTBOOK OF ZOOLOGY 



after leaving the spinal cord, they are covered on the ventral side by 

 a large calcareous body, the periganglionic gland, or "gland of 

 Swammerdam. " 



The first spinal nerve arises between the first and second vertebrae, 

 the second between the second and third vertebrae, and so on until 

 the tenth, which is small and emerges from the urostyle near its an- 

 terior end. These nerves frequently send branches to preceding or 

 succeeding nerves to form plexuses. Two large plexuses in particular 

 are present. Branches from the first and third nerves join with the 

 large second nerve to form the trachial plexus, which supplies nerves 

 to the muscles of the forelimbs and shoulder. Nerves number seven, 

 eight, and nine fuse to form the large sciatic plexus which supplies 

 the sciatic nerve to the hind leg. 



Sympathetic Nervous System. — From the first sympathetic gan- 

 glion, nerves are given off which form a cardiac plexus on the heart. 

 Another plexus, formed primarily from nerves of the third, fourth, 

 and fifth sympathetic ganglia, is the solar plexus on the dorsal surface 

 of the stomach. In addition, numerous ganglia are scattered through- 

 out the tissues of the body, all being connected by sympathetic nerve 

 fibers and finally communicating with the sympathetic trunks. The 

 cooperation of certain cranial and spinal nerves with the sympa- 

 thetic in relation to the involuntary actions of a number of the vital 

 internal organs is referred to as the autonomic function. 



The Sense Organs 



The olfactory sacs, or nasal chambers, are located internal to the 

 external nares. The median portion of the nasal chamber is lined 

 with olfactory epithelium which contains sense cells possessing proto- 

 plasmic processes known as olfactory hairs on their free ends. These 

 olfactory hairs are stimulated by chemical substances present in the 

 air and pass the stimuli received through the olfactory cells to the 

 olfactory nerves. 



The degree to which the sense of smell is used by amphibians is not 

 known. It is likely, however, that it may cause the frog at times to 

 approach objects and may serve to test the food substances it takes 

 into its mouth. 



The eyes lie in cavities, or orbits, on the dorsolateral sides of the 

 head. The exposed portion of the eyeball is covered by a transparent 

 membrane, the cornea, which is continuous with the opaque connec- 



