300 ANIMAL BIOLOGY 



Though separated from one another by a deep median 

 cleft or fissure they are broadly united by a mass of trans- 

 verse fibers, the corpus callosum. The surface of the 

 hemispheres is thrown into numerous folds or convolutions 

 separated by fissures. The outer part, or cortex, of the 

 hemispheres is composed of gray matter, which consists 

 mainly of ganglion cells and their interconnecting processes. 

 There are numerous fibers which pass from the cortex 

 to certain large ganglionic masses at the base of the brain 

 and these in turn are connected with the spinal cord and 

 with various cranial nerves. Then there are numerous 

 fibers which run from one part of the cortex to the other 

 so that the whole structure may be regarded as a great 

 mass of nerve cells closely united by connecting fibers and 

 intimately connected with other parts of the brain, with 

 the spinal cord, and thence with other parts of the body. 

 Below the posterior part of the cerebral hemispheres is 

 the cerebellum, an organ which probably has to do with the 

 control or coordination of bodily movements, but of whose 

 precise functions comparatively little is known. 



. The lowest part of the brain, the bulb or medulla, may 

 be regarded as an enlargement of the spinal cord. From 

 this part arise several pairs of nerves which are mainly 

 distributed to the head, face and neck; one pair, however, 

 the vagus nerve, sends branches to the lungs, heart and 

 organs of digestion. This nerve has an important influence 

 upon the beating of the heart and the movements of 

 respiration. Destruction of a certain center in the me- 

 dulla results in death, because respiratory movements are 

 stopped and the animal dies of suffocation. An animal 

 can live without a cerebrum, although he would be a very 

 stupid sort of creature, but the medulla is absolutely 

 essential to life. 



When the cerebral hemispheres are taken out of a pigeon 



