80 



THE NERVOUS SYSTEM. 



cular and is divided by certain fissures, of which the 

 most important are the anterior and posterior median, 

 the latter being rather a dividing line or septum. By 

 them it is divided into halves connected by a small 

 band in the middle called the commissure. The white 

 matter is exterior to the gray and is divided by it into 

 four columns, which again are divided into tracts ac- 

 cording to certain groups of nerves that travel through 



FIG. 28. Different views of a portion of the spinal cord from the cervical 

 region, with the roots of the nerves. In A the anterior surface of the specimen 

 is shown, the anterior nerve root of its right side being divided; in B a view 

 of the right side is given; in C the upper surface is shown; in D the nerve roots 

 and ganglion are shown from below: 1, the anterior median fissure; 2, posterior 

 median fissure; 3, anterior lateral depression, over which the anterior nerve roots 

 are seen to spread; 4, posterior lateral groove, into which the posterior roots are 

 seen to sink; 5, anterior roots passing the ganglion; 5', in A, the anterior root 

 divided; 6, the posterior roots, the fibers of which pass into the ganglion, 6; 7, the 

 united or compound nerve; 7', the posterior primary branch seen in A and D to 

 be derived in part from the anterior and in part from the posterior root. (Allen 

 Thomson.) 



them. The most important tract is the direct pyram- 

 idal tract in the anterior column. The gray matter is 

 arranged in the form of a letter H practically, consist- 

 ing of two lateral halves, more or less crescentic in out- 

 line, connected by a narrow band, the gray commissure. 

 Each half is divided into two horns, the anterior, to- 

 ward the front of the cord, and the posterior, toward 

 the back, the former being generally much thicker and 

 heavier than the latter. The structure of the gray and 



