SPINAL NERVES. 85 



cera. They consist of a dense, investing, fibrous membrane, containing in 

 its interior gray or vesicular cells, among which are found white and gela- 

 tinous nerve fibres. They may be regarded as independent nerve centres. 



Structures of Nerves. Nerves are rounded or flattened cords extend- 

 ing from the centres to the periphery ; they are surrounded externally by a 

 sheath, the neurilemma, composed of fibrous and elastic tissue forming a 

 stroma, in which blood vessels ramify, from which the nerves derive their 

 nourishment. 



A Nerve consists of a greater or less number of ultimate nerve filaments ', 

 separated into bundles by fibrous septa given off from the neurilemma. The 

 nerve filaments pursue an uninterrupted course, from their origin to their 

 termination; branches pass from one nerve trunk into the sheath of 

 another, but there is no anastomosis or coalescence with adjoining nerve 

 fibres. 



A Plexus is formed by a number of branches of different nerves inter- 

 lacing in every direction, in the most intricate manner, but from which 

 fibres are again given off to pursue their independent course, e.g., brachial, 

 cervical, lumbar, sacral, cardiac plexuses, etc. 



SPINAL NERVES. 



Origin. The spinal nerves are thirty-one in number on each side of the 

 spinal cord, and arise by two roots, an anterior and posterior, from the 

 anterior and posterior aspects of the cord respectively : the posterior roots 

 present near their emergence from the cord a small ganglionic enlargement ; 

 outside of the spinal canal the two roots unite to form a main trunk, which 

 is ultimately distributed to the skin, muscles and viscera. 



The Function of the Anterior Roots is to transmit motor impulses 

 from the centres outward to the periphery. Irritation of these roots, from 

 whatever cause, excites convulsive movements in the muscles to which they 

 are distributed ; disease or division of these roots induces a condition of 

 paresis or paralysis. - 



The Function of the Posterior Roots is to transmit the impressions 

 made upon the periphery to the centres in the spinal cord, where they 

 excite motor impulses ; or to the brain, in which they are translated into 

 conscious sensations. Irritation of these roots gives rise to painful sensa- 

 tions ; division of the roots abolishes all sensation in the parts to which 

 they are distributed. 



The ganglion on the posterior root influences the nutrition of the sensory 



