A STUDY OF THE NERVOUS SYSTEM 



261 



between the number of nerves and the number of vertebrae 

 in each region : 



Distribution of Nerves. Five of the nerves on each side 

 that come from the cervical enlargement, after uniting more 

 or less with each other, pass down the arm, supplying its 

 various muscles and sense organs. To the hips and the 

 legs, likewise, are distributed most of the nerves from the 

 lumbar enlargement. All the spinal nerves, after their exit 

 from the spinal column, divide into smaller and smaller 

 branches, and these reach all parts of the trunk and the 

 appendages (see Fig. 115). 



Origin of the Nerves. Each spinal nerve arises from the 

 cord by two so-called roots. From the dorsal surface of the 

 spinal cord on each side, a white nerve trunk passes outward 

 to form the dorsal root of the nerve. Strands of fibers that 

 originate in the gray ventral horn unite to form the ventral 

 root. These two roots come together within the bony cavity 

 of the spinal column, forming the spinal nerve which we 

 have followed out through a hole between the vertebrae to 

 its destination in the tissues (see Figs. 116 and 117). 



Structure of a Spinal Nerve. In the cross section of a 

 spinal nerve one sees that the whole nerve trunk is sur- 

 rounded by connective tissue. Within this outside sheath 

 the nerve fibers are collected into bundles, and each bundle 

 is inclosed by a covering of connective tissue, called per'i- 



