206 ELEMENTS OF MAMMALIAN ANATOMY. 



pia mater is prolonged into the anterior fissure, but not 

 into the posterior fissure. Slightly laterad of the anterior 

 median fissure issue the anterior roots of the spinal 

 nerves, and at about the same distance from the posterior 

 median fissure are the posterior roots of the spinal nerves. 

 These two roots unite about one centimeter from their 

 origin (Fig. 90). On the posterior or sensory root is a 

 small ganglion located very near the junction of the two 

 roots. The common spinal nerve, formed by the union 

 of the two roots, almost immediately divides into four 

 branches, one of which, the dorsal, supplies the muscles 

 and skin along the vertebral column; a second, the ven- 

 tral branch, supplies the limbs or intercostal spaces; 

 while the other two branches, rami communicantes, join 

 the adjacent ganglion of the sympathetic cord (Fig. 103). 

 Each of the four branches contains both motor and sen- 

 sory fibers, or, in other words, fibers from both roots. 



The exit and entrance of the nerve roots divide the 

 white matter of the lateral half into three columns named, 

 according to their location, the anterior column, the 

 lateral column, and the posterior column. The first is 

 ventral to the anterior nerve roots, the last is dorsal to 

 the posterior nerve roots, while the lateral column is 

 between the roots. 



The elements of the spinal cord, like those of the 

 brain, are of two kinds the cells composing the gray 

 matter and the fibers composing the white matter. 

 While in the brain the gray matter is largely on the 

 surface, forming the cortex, in the cord it occupies the 

 central region. A canal, the canalis centralis, about one- 

 half a millimeter in diameter, extends throughout the 

 cord, opening into the fourth ventricle of the brain. A 

 cross-section of the cord shows the gray matter arranged 

 in the shape of a letter H . The ventral columns of gray 



