ANATOMY OF THE NERVOUS SYSTEM. 163 



tips of each crescent are called its horns or cornua, and the 

 ventral, or anterior cornu, on each side is thicker and larger 

 than the posterior. In the cervical and lumbar enlargements 

 the proportion of white to gray matter is greater than else- 

 where; and as the cord approaches the medulla oblongata its 

 central gray mass becomes irregular in form and begins to 

 break up into smaller portions. If lines be drawn on the 

 transverse section of the cord from the tip of each horn of the 

 gray matter to the nearest point of the surface, the white sub- 

 stance in each half will be divided into three portions: one 

 between the anterior fissure and the anterior cornu, and 

 called the anterior white column ; one between the posterior 

 fissure and the posterior cornu, and called the posterior white 

 column ; while the remaining one lying in the hollow of the 

 crescent and between the two horns is the lateral column : the 

 anterior and lateral columns of the same side are frequently 

 named the antero -lateral column. A certain amount of white 

 substance crosses the middle line at the bottom of the ante- 

 rior fissure; this forms the anterior white commissure. There 

 is no posterior white commissure, the bottom of the posterior 

 fissure being the only portion of the cord where the gray sub- 

 stance is uncovered by white. Running along the middle 

 of the gray commissure, for the whole length of the cord, is 

 a tiny channel, just visible to the unaided eye; it is known as 

 the central canal (canalis centralis). 



The Spinal Nerves. Thirty-one pairs of spinal nerve- 

 trunks enter the neural canal of the vertebral column through 

 the intervertebral foramina (p. 71). Each divides in the fora- 

 men into a dorsal and ventral portion known respectively 

 as the posterior and anterior roots of the nerve (6 and 5, Fig. 

 71), and these again subdivide into finer branches which are 

 attached to the sides of the cord, the posterior root at the 

 point where the posterior and lateral white columns meet, 

 and the anterior root at the junction of the lateral and anterior 

 columns. At the lines on which the roots are attached there 

 are superficial furrows on the surface of the cord. On each 

 posterior root is a spinal ganglion (6', Fig. 71), placed just be- 

 fore it joins the anterior root to make up the common nerve- 

 trunk. Immediately after its formation by the mixture of 

 fibres from both roots, the trunk divides (D, Fig. 71), into 

 a posterior primary, an anterior primary, and a communi- 

 cating branch. The branches of the first set go for the most 



