86 



HUMAN PHYSIOLOGY 



descend within it for some distance below the extremity 

 of the cord. 



FIG. 20. At the left is shown the shape of the 

 spinal cord as seen from behind. Two enlarge- 

 ments are noticeable; the upper is the region from 

 which the arms are supplied, the lower stands in 

 relation with the legs. Two vertebrae are sketched 

 near the middle of the cord to show how it is en- 

 closed by their arches. 



At the right is a cross-section of the cord with its 

 dorsal side upmost and the H of gray matter rep- 

 iresented. (d.c.), (Z.c.), and (B.C.) are the dorsal, 

 lateral, and ventral columns of the white matter. 



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 con- 

 fines of the vertebra?. 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 ventraJ 

 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 



