SPINAL NERVES. 123 



root ; the posterior roots being the larger, and having a gan- 

 glion. Immediately beyond the ganglion, the two roots 

 unite into a single mixed nerve, which passes out of the 

 spinal canal by the intervertebral foramen. The nerve thus 

 constituted is endowed with both motor and sensory prop- 

 erties. It divides outside of the spinal canal into two 

 branches, anterior and posterior, both containing motor and 

 sensory filaments, which are distributed respectively to the 

 anterior and posterior parts of the body. The anterior 

 branches are the larger, and supply the limbs and all parts 

 in front of the spinal column. 



The anterior branches of the four upper cervical nerves 

 form the cervical plexus, and the four inferior cervical nerves, 

 with the first dorsal, form the brachial plexus. The anterior 

 branches of the dorsal nerves, with the exception of the first, 

 supply the walls of the chest and abdomen. These nerves 

 go directly to their distribution, and do not first form a 

 plexus, like most of the other spinal nerves. The anterior 

 branches of the four upper lumbar nerves form the lumbar 

 plexus. The anterior branch of the fifth lumbar nerve and 

 a branch from the fourth unite with the anterior branch of 

 the first sacral, forming the lumbo-sacral nerve, and enter 

 into the sacral plexus. The three upper anterior sacral 

 nerves with a branch from the fourth form the sacral plexus. 

 The greatest portion of the fourth anterior sacral is distrib- 

 uted to the pelvic viscera and the muscles of the anus. The 

 fifth anterior sacral and the coccygeal are distributed about 

 the coccyx. 



The posterior branches of the spinal nerves are very sim- 

 ple in their distribution. With one or two exceptions, which 

 have no great physiological importance, these nerves pass 

 backward from the main trunk, divide into two branches, 

 external and internal, and their filaments of distribution go 

 to the muscles and integument behind the spinal column. 



It is further important to note, as we shall have occasion 

 to do more particularly in connection with the great sym- 



