The Frog: A Representative Vertebrate 71 



The central nervous system is protected with a covering of three 

 membranes, the meninges. The outer heavy dura mater and the more 

 deHcate pia mater cover the entire system. In addition an anterior 

 choroid plexus covers a small opening in the dorsal part of the dien- 

 cephelon, and a posterior choroid plexus covers the fourth ventricle 

 of the medulla. The choroid plexus contains a rich network of blood 

 vessels. Together these membranes provide for the protection and 

 nourishment of the central nervous system. 



SPINAL NERVES' I 2 3 

 OtrACTORY LOBE 

 CEREBRUM 

 DIENCEPHALON 

 .OPTIC LOBE 

 MEDOLLA. 



4 J e 7 e e 10 



Fig. 19. — Dorsal view of nervous system of bullfrog. 



The Peripheral Nervous System. — This system consists of the 

 nerves which extend outward from the central nervous system to all 

 parts of the body. Eleven pairs of cranial nerves arise from the brain 

 and ten spinal nerves arise from the spinal cord. These nerves are seg- 

 mentally arranged and appear as small white threads. 



The Autonomic System. — The autonomic nervous system con- 

 trols the activites of the visceral organs. Principally it consists of 

 two slender nerve strands along the dorsal wall of the coelom, passing 

 along on either side of the dorsal aorta. Each strand has ten ganglia 

 or nerve cell masses from which nerves arise that connect to the vis- 

 cera and central nervous system. 



Sense Organs. — Associated with the nervous system are the sense 

 organs by which the animal receives stimuli from the outer environ- 

 ment. Many simple sense organs are located in the skin and through- 

 out the visceral organs. In addition, more complicated ones such as 

 the taste buds, nose, ear, and eye are present. 



THE ENDOCRINE SYSTEM 



The other important coordinating system of the frog is the endo- 

 crine system which is known as the chemical coordinating system. The 



