THE NERVOUS SYSTEM 209 



about -g-Q^ of an inch in diameter, termed the nerve-fibres; 

 and (2) the gray substance, composed of grayish-red, or ashen- 

 colored cells, of various sizes, generally possessing one or more 

 off-shoots, which are continuous with the nerve-fibres just men- 

 tioned. 



5. The gray, cellular substance constitutes the larger por- 

 tion of those important masses which bear the name of nert-oii* 

 centres and ganglia* (from ganglion, a knot), in which all the 

 nerve-fibres *"irriite? These white nerve-fibres are found com- 

 bined together in long and dense cords, called tierces (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 animal functions comprises the brain, the spinal 

 cord, and the nerves which are derived therefrom; these are, 

 together, called the cerebro-spinal system (Fig. 54) ; 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 nervous 

 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 commencement or expansion of the spinal cord, termed 

 the medulla oblongata, or oblong marrow. 



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

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



5. Nervous j;entres and ganglia ? Nerves? What do they serve? Cerebro-spinal 

 system"? 



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

 at the base ? 



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

 branes of the brain ? Blood sent to the brain ? 



