150 THE NERVOUS SYSTEM. 



colored cells, of various sizes, generally possessing one or 

 more off-shoots, which are continuous with the nerve- 

 fibres just mentioned. 



5. The gray, cellular substance constitutes the larger 

 portion of those important masses, which bear the name of 

 nervous centres and ganglia (from ganglion, a knot), and 

 in which all the nerve-fibres unite. These white nerve- 

 fibres are found combined together in long and dense 

 cords, called nerves (from neuron, a cord), which serve to 

 connect the nervous centres with each other, and to place 

 them in communication with all the other parts of the 

 body which have sensibility or power of motion. That 

 part of the nervous system which is concerned in the ani- 

 mal functions, comprises the brain, the spinal cord, and the 

 nerves which are derived therefrom; these are, together, 

 called the cerebro-spinal system (Fig. 40) ; while that other 

 set of organs, which presides over, and regulates the vege- 

 tative functions, is called the sympathetic system of nerves. 



6. The Brain. The brain is the great volume of nerv- 

 ous tissue that is lodged within the skull. It is the largest 

 and most complex of the nervous centres, its weight, in the 

 adult, 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 commence- 

 ment or expansion of the spinal cord, termed the medulla 

 oUongata 9 or oblong marrow. 



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



5. Nervous centres and ganglia? Nerves? What do they serve? Cerebro- 

 epinal system? 



6. Location of the brain ? Its weight ? Its shape ? Of what it consists ? What 

 or-ran* at the base ? 



7. The tissue of the brain ? What, therefore, is required ? Blows on the head ? 

 Membranes of the brain ? Blood sent to the brain ? 



