226 THE NERVOUS SYSTEM 



it inward, we should have found that it followed a course 

 similar to that of the motor fibres, perhaps even in the 

 same nerve and nerve trunks, until it reached the neural 

 canal of the spinal column. Here, instead of passing 

 through the ventral root of its spinal nerve, it passes 

 into the dorsal root and thence by a short right-angled 

 branch to the nerve cell of which it is a prolongation. 

 This nerve cell is situated in an enlargement, or ganglion, 

 of the dorsal root of the spinal nerve, which also con- 

 tains the other cells belonging to the similar fibres of 

 this spinal nerve. Each of these ganglion cells sends 

 inward to the spinal cord a branch, which, after enter- 

 ing the spinal cord, divides into numerous branches. 

 Each ganglion cell with its branches also forms a 

 neuron. These neurons have quite a different function 

 from the motor neurons, in that they are responsible not 

 for our activities but for our sensations. Along them 

 travel the nervous impulses which arise in the skin and 

 which result in our sensations of touch, pressure, heat 

 and cold. They are therefore called sensory neurons. 

 If sensory neurons are injured, as for example by cut- 

 ting the dorsal root of a spinal nerve, there is an imme- 

 diate loss of sensation in the part to which the cut nerves 

 go. That part of the skin may even be pricked or 

 burned, but the person will feel nothing. 



Direction of nervous impulses. In the case of sen- 

 sation, the nervous impulses start in the skin and pass 

 inward along the sensory nerves to the spinal cord and 

 thence to the brain. In the case of muscle and gland 

 stimulation, on the contrary, the nervous impulses orig- 

 inate in the brain and spinal cord and pass along the 

 motor nerves outward. 



Spinal cord. The spinal cord is thus seen to con- 

 tain the inner ends of both the motor and sensory neu- 



