THE NERVOUS SYSTEM 305 



number, arising directly from the brain and passing out of 

 the cranium to supply, chiefly, the organs of the head; Fig. 157. 

 The muscles of the head are controlled by them, sensations 

 from the face, the nose, the eye, the ear and the tongue are 

 received through them. Some of the more important of these 

 we shall consider in the following chapter. 



The Spinal Nerves, i Between the neural arches of each 

 two vertebrae enough space is left for a nerve of considerable 

 size to pass from the spinal cord on each side. There are 

 thirty-one of these spinal nerves on Posterior Root 



each side of the cord. Five of them 

 unite to make up the brachial plexus, 

 i.e. the nerve combination which 

 supplies the arm; four form the lumbar 

 plexus and thence pass down each leg 

 as one nerve; Fig. 153. The rest supply FlG - 158. A BIT OF 

 the numerous organs of the neck and THE SPINAL CORD 



i T-I- i ro i j.i Showing the method of 



trunk proper. Figure 158 shows that a origin of the spinal nerve s 

 spinal nerve does not leave or enter the b y two roots - 

 cord in one place as a branch grows out of a tree, but arises 

 by two roots, one being continuous with the gray matter in 

 the anterior part of the cord and the other with that in the 

 posterior part. These two roots join to form one nerve; 

 before their junction, however, a swelling, a nerve ganglion, 

 occurs on the posterior root. 



The precise function of these two roots has been ascertained 

 by experiments upon animals. If, for example, the posterior 

 roots of all the nerves going to some one organ, e.g. the leg, 

 have been cut, it is found that nothing touching the leg, not 

 even a burn, is felt in the least, but it is still possible for the 

 animal to move the leg or any part of it. This result shows 

 that all sensory impulses, all messages having to do with 

 feeling, as we say, pass from the leg into the spinal cord over 

 the posterior roots of the spinal nerves. The function of the 



