CHAPTER XXXI 

 THE BRAIN 



518. General structure. The brain is the part of the 

 central nervous system which can originate orders in dis- 

 tinction from the spinal cord, which acts only in response 



Brain of a frog ; top view (X 10). 

 a cerebrum. 

 b optic tubercles. 

 c cerebellum. 

 d medulla. 

 e upper end of spinal cord. 



Brain of a hen ; side view (X 

 a cerebrum. 

 b optic nerve. 

 c optic tubercle. 

 d medulla. 

 e cerebellum. 

 / spinal cord. 



to impulses brought to it. In reptiles, toads, and frogs, it 

 is very simple in structure, but yet contains parts corre- 

 sponding to all the parts of the brain of man. In them the 

 spinal cord swells out to form a cone-shaped body called 

 the medulla oblongata. Above it there is a small flat 



OV. PHYSIOL. 19 289 



