SKELETAL MUSCLE AND THE NERVOUS SYSTEM 85 



Origin of Nerve-impulses. We can now proceed to 

 investigate the place of origin of the nerve-impulses 

 by which muscles are thrown into action. We have 

 pictured a cluster of muscle fibers whose end-plates 

 are supplied by the branches of a single nerve fiber. 

 Whence does this fiber come? The answer will depend 

 upon the position of the muscle in the case so we will 

 assume a definite example. Suppose that it is the biceps 

 of the arm. A motor fiber ending in this muscle could 

 be traced back to the intricate commingling of nerves 

 in the neck to which we give the name of the cervical 

 plexus. The fiber could, theoretically at least, be 

 shown to have come by an unbroken course through 

 this maze and to have originated in the spinal cord near 

 the same level. 



If we had chosen a muscle of the head for instance, 

 the masseter which acts upon the lower jaw we should 

 have found the selected fiber to have come through one 

 of the openings of the skull and to have arisen in the 

 lower part of the brain. The motor fibers for the leg 

 muscles come from the lower levels of the spinal cord. 

 Since all the skeletal muscles excepting those of the 

 head derive their motor supply from the spinal cord 

 we must attend now to some of the features of this 

 part of the nervous axis. 



The Spinal Cord and the Spinal Nerves. It has 

 been previously pointed out that the spinal cord occu- 

 pies a canal formed by the arches of the vertebrae. 

 The cord is continuous with the brain above, a large 

 opening in the base of the skull providing for the union. 

 Protective membranes, the meninges, with more or less 

 included fluid, envelop the cord. The spinal nerves 

 spring from it in pairs; thirty-one nerves on each side. 

 They go out through notches in the bones, one pair 

 between each two vertebrae. The cord is not so long 

 as the canal in which it is lodged; the result is that the 

 nerves which are to leave the lower end of the canal 



