704 EVOLUTION OF ANIMALS Part V 



Nervous System — Cellular Control, Perception. Nervous and en- 

 docrine systems cooperate, with the nervous system taking the lead in quick 

 actions. The nervous system is divided into three closely associated divisions: 

 the cerebrospinal, brain and spinal cord; the peripheral, all the nerves which 

 extend to and from the brain and cord and connect them with the sense organs, 

 muscles and outer parts of the body; and the autonomic (involuntary) division, 

 the nerves that carry messages to and from the digestive, respiratory, and 

 circulatory systems and the glands (Fig. 34.21). All three divisions work to- 

 gether to make a unified animal. 



Cerebrospinal Division. The narrow cranium and the bony tube made by 

 the vertebrae form a first line of defense for the brain and cord. Within this 

 are other covers, the meninges. The colorless cerebrospinal fluid circulates 

 slowly about the cord, within its central canal, and through the ventricles of 

 the brain. Oxygen is supplied from this fluid as well as from the blood. 



Spinal Cord. The spinal cord is a tube with relatively thick walls and a 

 minute central canal which continues into the brain where it widens into the 



'Spinal cord 



Sympathetic 

 trunk and 



Sciatic nerve 



Sciatic plexia 



Fig. 34.21. Nervous system of the frog, ventral view; the brain and cord and 

 their branches; the sympathetic nerve trunks (part of the autonomic system) lie 

 on either side of the cord and the branches join the spinal nerves. Cranial nerves, 

 Roman; spinal nerves, Arabic. (Courtesy, Wolcott: Animal Biology, ed. 3. New 

 York, McGraw-Hill Book Co., 1946.) 



