118 HUMAN PHYSIOLOGY. 



portion, the postero-internal column, or the column of Goll, bordering the 

 posterior median fissure, and (<5) an external portion, the poster o- external 

 column, the column of Burdach, lying just behind the posterior roots. 

 They are composed of long and short commissural fibres which connect 

 together different segments of the spinal cord. 



Structure of the Gray Matter. The gray matter, arranged in the 

 form of two crescents, presents an anterior and posterior horn. It is made 

 up of a delicate network of fine nerve fibres (axis cylinders), supported by 

 a connective tissue frame work of nucleated nerve cells, which in the anterior 

 horns are large and multipolar, and connected with the anterior roots of 

 spinal nerves ; in the posterior horns the nerve cells are smaller, and situated 

 along the inner margin, and in the caput cornu. Small cells are also found 

 in the posterior vesicular columns, and in the intermediary lateral tract. 



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 

 nerve; for if the nerve be separated from the ganglion, it undergoes 

 degeneration in the course of a few days, in the direction in which it 

 carries impressions, i. e., from the periphery to the centres; if the nerve be 

 divided between the ganglion and the cord, the central end only undergoes 



