THE NERVOUS SYSTEM 121 



cerebrum, the cerebellum, the pons, and the medulla oblongata. 

 Completely enveloping these parts and continued onto the spinal 

 cord are three membranes or meninges. 



The dura mater is the most external. It is in the form of a 

 thick, tough, fibrous covering, bluish white in color. It hues the 

 cranial cavity and extends backward to form a tube-like sheath 

 for the spinal cord. 



The arachnoid is the middle coat. It is a delicate serous mem- 

 brane. Between its two layers is a space containing the cerebro- 

 spinal fluid. This fluid acts as a sort of water-jacket to take the 

 jar and pressure off the sensitive nerve structures which it encloses. 



The pia mater closely covers and adheres to both the brain and 

 spinal cord. It contains numerous small blood-vessels for these 

 structures. 



The cerebrum, or large brain, occupies the anterior part of the 

 cranial cavity. It is somewhat egg shaped, and is divided by a 

 deep median fissure into two hemispheres. Externally, the sur- 

 face is thrown up into well-defined convolutions. The outer part 

 is composed of gray matter. In the center is found a core of white 

 matter. An irregular cavity, termed the lateral ventricle, may be 

 seen in the interior of each hemisphere. The olfactory bulb is the 

 enlargement which projects from each of the two anterior poles. 



The cerebellum, or small brain, is lodged in the posterior fossa 

 of the cranium. It resembles a globe in shape and differs from the 

 cerebrum in having a finer marked exterior. It is divided into the 

 median vermis and two lateral hemispheres. For the most part it 

 is composed of gray substance, but a core of white material resem- 

 bling the branchings of a tree may be seen when the cerebellum is 

 sectioned. 



The pons (Varolii) is a bridge-like affair, which lies between the 

 medulla and the cerebral peduncles. It connects the former with 

 the cerebrum, and also the right and left portions of the cerebellum. 



The medulla oblongata is the connecting link between the pons 

 and the spinal cord. It differs from the brain proper in having 

 the white substance externally. 



The cranial nerves arise directly from the brain. There are 

 twelve pairs, which have been numbered and named to identify 

 them. The names indicate the functions or the parts to which 

 they pass. 



The first or olfactory nerve supphes the mucous membrane of 



