THE NERVOUS SYSTEM 221 



being about fifty ounces, or one-fortieth of that of the whole 

 body. The shape of the brain is oval, or egg-shaped,- with one 

 extremity larger than the other, which is placed posteriorly in 

 the skull, to the concavity of which it very closely conforms. 

 The brain consists chiefly of two parts ; the cerebrum, or brain 

 proper, and the cerebellum, or "little brain." In addition to 

 these, there are several smaller organs at the base, among 

 which is the commencement or expansion of the spinal cord, 

 termed the medulla oblongata, or oblong marrow. 



8. The tissue of the brain is soft and easily altered in shape 

 by pressure; it therefore requires to be placed in a well-pro- 

 tected position, such as is afforded by the skull, or cranium, 

 which is strong without being cumbrous. In the course of 

 an ordinary lifetime, this bony box sustains many blows with 

 little inconvenience ; while, if they fell directly upon the brain, 

 they would at once, and completely, disorganize that structure. 

 Within the skull, the brain is enveloped by certain membranes, 

 which at once protect it from friction and furnish it with a 

 supply of nutrient vessels; they are called the arachnoid, or 

 "spider's web," the dura mater, and the pia mater, or the 

 "tough" and "delicate coverings." These membranes also 

 cover the spinal cord. 



9. The dura mater is the outer covering. It is very strong 

 and tough, being composed of white fibrous and elastic tissue. 

 Next comes the arachnoid, which consists of two layers of flat 

 cells, one next the dura mater, and the other next the pia 

 mater. Between these two layers is contained a small quan- 

 tity of watery cerebro-spinal fluid. This fluid is also found on 

 the pia mater in the folds of the brain surface. The pia mater 

 adheres closely to the surface of the brain and spinal cord. It 

 is less tough and less closely woven with fibrous and elastic 

 tissue than the dura mater. It contains a network of blood- 

 vessels which nourish the brain. The supply of blood sent to 

 the brain is very liberal, amounting to one-fifth of all that the 



8. The tissu* ef the brain ? What, therefore, is required ? Blows on the head ? Mem- 

 branes of th brain ? Blood sent to the brain ? 



9. The 4ura mater? Arachnoid? Cerebro-spinal fluid ? The pia mater? 



