160 THE HUMAN BODY. 



the anterior fissure; this forms the anterior white commis- 

 sure. There is no posterior white commissure, the bottom 

 of the posterior fissure being the only portion of the cord 

 where the gray substance 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 cen- 



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 di- 

 vides in the foramen into a dorsal and ventral portion 

 known respectively as the posterior and anterior roots of 

 the nerve (6 and 5, Fig. 64), 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 late- 

 ral white columns meet, and the anterior root at the junc- 

 tion 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. 64), placed just before it joins the an- 

 terior root to make up the common nerve-trunk. Imme- 

 diately after its formation by the mixture of fibres from 

 both roots, the trunk divides into a small posterior primary 

 find a larger anterior primary branch (7' 7 D, Fig. 64). 

 The former branches of the spinal nerves go for the most 

 part to the skin and muscles on the back, while the anterior 

 primary branches form a series of plexuses from which the 

 nerves for the sides and ventral region of the neck and 

 trunk, and for the limbs, arise. 



The various spinal nerves are named from the portions 

 of the vertebral column through the intervertebral foramina 

 of which they pass out; and as a general rule each nerve is 

 named from the vertebra in front of it. For example the 

 nerve passing out between the fifth and sixth dorsal verte- 

 brae is the "fifth dorsal" nerve, and that between the last 

 dorsal and first lumbar vertebrae, the "twelfth dorsal." 

 In the cervical region, however, this rule is not adhered to. 

 The nerve passing out between the occipital bone and the 



