70 The Vertebrate Organ Systems 



with the spinal cord. The medulla provides not only the necessary 

 nerve tracts for connecting the brain with the cord, but also has nerve 

 centers for the control of various vital activities. 



Fig. 18. — Brain of bullfrog. A, Dorsal view; B, ventral view. 



The center of the cord is a hollow passageway or central canal 

 which connects with spaces or ventricles in the brain. Two ventricles 

 are found in the cerebrum, one in each lobe. These join with a third 

 ventricle in the diencephalon which in turn, by means of a narrow 

 aqueduct, connects to the fourth ventricle of the medulla. This latter 

 is continuous with the central canal of the cord. This entire canal 

 system is filled with a cerebrospinal fluid which equalizes pressure 

 inside the central nervous system and provides nourishment for the 

 internal cells. 



The spinal cord passes as a long tapering structure through the 

 neural arches of the vertebrae. It terminates as a threadlike filament 

 in the urostyle. Ventral and dorsal fissures may be seen in the cord. 



