THE NERVOUS SYSTEM 117 



the stomach. It is the agent, therefore, by which the 

 voice is produced, the air passages guarded against ir- 

 ritants and excited to expel them; the heart regulated, 

 glands of the stomach and the pancreas incited to activ- 

 ity and the musculature of the swallowing and digestive 

 organs stimulated to perform their functions. 



The spinal nerves, unlike the cranial, always spring 

 from the spinal cord by two roots, an anterior, motor, 

 connected with the anterior column and adjacent gray 

 horn, and a posterior sensory connected with the poste- 

 rior columns and the gray matter of the posterior horn. 

 After emerging there is formed, on the posterior root 

 only, a ganglion from whose cells the posterior root 

 really springs. This ganglion is the trophic center of 

 this root; i.e., that mass of cells which is so essential to 

 the health and activity of a tissue that without it death 

 of the tissue and degeneration will occur. This is proved 

 by cutting one nerve between the ganglion and the cord 

 and another between the ganglion and periphery, in the 

 latter case only will the nerve degenerate. Beyond the 

 ganglion the two roots unite to form the spinal nerves 

 as we dissect them. In all regions except the thoracic, 

 spinal nerves thus formed communicate more or less in- 

 timately with one another to form anatomic plexuses, 

 from which the ultimate branches of distribution are de- 

 rived. In these plexiform communications there is a re- 

 distribution of fibers in such a way that some nerves 

 emerge which are entirely afferent, some entirely effer- 

 ent, but most are, like the parent trunks, mixed. The 

 plexuses are cervical, brackial, lumbar and sacral and 

 coccygeal. 



The cervical plexus supplies the skin over the neck, 

 upper part of the chest, back of the head and thorax and 

 muscles in the same region. 



