SKELETAL MUSCLE AND THE NERVOUS SYSTEM 87 



inward, afferent fibers. Not all the efferent fibers 

 emerging from the cord in the ventral roots are of 

 the motor type which we are just now describing, but 

 a large proportion of them are of this sort. 



White and Gray Matter. When a cross-section of 

 the spinal cord is examined with the naked eye two 

 kinds of tissue are apparent in it. These could be 

 recognized long before the workers with the modern 

 microscope had resolved them into their elements. 

 They have been known respectively as the white and 

 the gray matter. The two exist not only in the cord 

 but in the brain. In the cord the gray matter is found 

 in all sections in a form which has usually been likened 

 to the letter H. A diagram will show how, by its 

 presence, it breaks the white matter into three col- 

 umns in either half of the cord, the dorsal, lateral, and 

 ventral (Fig. 18). 



The white matter of the cord and brain is nearly 

 identical with the substance of the nerves. That is to 

 say, it is composed of conducting fibers, closely packed 

 in parallel bundles. If it were not for the gray matter 

 in the spinal cord we should be justified in calling that 

 structure the greatest of all the nerves. In fact we 

 may call it so but we must add that it is more than this. 

 To find out why we must examine the gray matter. 



The structure of the gray matter is peculiarly intri- 

 cate and obscure. There is much disagreement with 

 reference to its finer features. Perhaps the first thing 

 which commands attention when it is viewed under the 

 microscope and compared with the white matter is an 

 apparent looseness of texture. There are nerve fibers 

 to be seen, but they are rather widely separated and 

 do not have a common direction. The most con- 

 spicuous bodies visible are the so-called nerve cells. 

 These are of several types. As a rule the following 

 characters may be recognized. The outline is singu- 

 larly broken, each cell having a number of processes. 

 Most of these can be shown to subdivide into extremely 



