222 



THE NERVOUS SYSTEM 



from it to all parts of the body. In the limbs and to a 

 less extent in the trunk, the larger nerves follow nearly 

 the same courses as the blood-vessels. They can be 

 readily distinguished from them because they appear as 

 solid white cords of considerable strength. 



Nerves. When the course of any nerve is traced from 

 its termination in a cell inward toward the brain, it is 

 seen to start as a very fine thread, or nerve fibre, visible 

 only under the microscope, which quickly joins with 

 similar fibres from other cells. Together they form the 

 bundle of fibres known as a nerve. As the nerve passes 

 inward, it constantly grows larger through the addi- 

 tion of many branches until a large nerve, or nerve 

 trunk, is formed. 



Spinal nerves. All the nerves in the body, with the 

 exception of those of the head, are finally united into 

 sixty-two nerve trunks, which on approaching the spinal 

 column are called spinal nerves. Two spinal nerves, 



one upon each side, pass 

 ventral horn . into the spinal column at 



each of its joints. These 

 constitute a pair of spinal 

 nerves, there being thirty- 

 one pairs in all. 



After passing between 

 the joints of the spinal 

 column, the spinal nerves 

 enter the neural canal 

 formed by the arches 



which spring from the bodies of the vertebrse. Here 

 they split into two parts, or roots, and enter the much 

 larger white tough flexible cord, the spinal cord, which 

 nearly fills the neural canal. The nerve fibres of 

 the spinal cord serve to connect the spinal nerves with 



Ventral root 



Ganglion 



Dorsal root 



Dorsal horn 



FIG. 119. Formation of a spinal nerve 

 (Testut.) 



