ADDITIONAL TOPICS IN HUMAN BIOLOGY 157 



means of these countless nerve fibers all parts of the body are put in 

 communication with the nerve centers (see Fig. 50). 



232. Cellular structure of the nervous system. If a section is 

 made of any part of the brain or spinal cord, two kinds of material, 

 known respectively as gray matter and white matter, may be distin- 

 guished. In the gray matter are countless nerve cells (Fig. 51) 

 which are very irregular in form. From most of the nerve cells 

 project numerous fine processes that look like tiny branching roots. 

 These bring the various nerve 



cells into communication with 

 each other. 



One fiber-like process, how- 

 ever, has fewer branches than 

 the others, and may be traced 

 for a considerable distance 

 from the cell body. This is 

 the beginning of a nerve fiber, 

 and it is the mass of nerve 

 fibers that make up most of 

 the white matter of the nerv- 

 ous system. 



233. Nerve impulses. We 



may compare nerve fibers to . 



FIG. 51. Nerve cell from spinal cord, 

 telegraph wires, and nerve 



impulses may be described as messages that pass along these fibers. 

 But in making these comparisons we must remember that telegraphy 

 and the action of the nervous system have, in all probability, little real 

 resemblance. We know that nerves transmit impulses at the rate of 

 about one hundred feet per second ; electricity travels thousands of 

 miles per second. Hence a nerve impulse cannot very closely re- 

 semble what we call a telegraph message. On the other hand, this 

 nerve impulse travels much too rapidly to be explained as a chemi- 

 cal or mechanical action. We must therefore admit our ignorance 

 of the real nature of the nervous impulse ; nor do we know the real 

 nature of the changes that take place in the nerve cells after receiv- 



