SKELETAL MUSCLE AND THE NERVOUS SYSTEM 85 



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 masse ter 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 

 descend within it for some distance below the extremity 

 of the cord. 



A single spinal nerve is made by the union of two 

 divisions, or roots, which-* spring .separately from the 

 surfaces of the cord, uniting .as they leave the confines 

 of the vertebrae. These roots are designated as dorsal 

 and ventral. The motor fibers which it is our present 

 interest to trace emerge from the cord in the ventral 

 roots. We may, anticipate a later discussion to the 

 extent of saying that the dorsal roots are composed 

 almost wholly of fibers whose service is to carry im- 

 pulses into the central axis. There are thus two great 

 classes of nerve fibers, those which bear impulses out- 

 ward, efferent fibers, and those which convey impulses 



