122 THE NERVOUS SYSTEM 



are concerned in the perception of painful stimuli and in the 

 recognition of extremes of temperature protopathic sensibility, 

 (ti) those which are concerned in the perception and localisa- 

 tion of light touch and in the recognition of intermediate 

 degrees of temperature epicritic sensibility ; (c) those which 

 are concerned in muscle and joint sense and in the apprecia- 

 tion of deep touch deep sensibility. The fibres which convey 

 deep sensibility run in the muscular branches and those convey- 

 ing joint sense are carried to their destination by the tendons. 

 When a nerve is completely divided proximal to the point 

 at which it gives off its first branch, the area of sensory loss 

 is usually smaller than the area of known anatomical supply, 

 because the areas supplied by adjoining sensory nerves over- 

 lap one another to a greater or less extent. This overlapping 

 is more marked in the case of protopathic and deep sensibility 

 than it is in the case of epicritic sensibility. On this account, 

 when a nerve, such as the median, is divided above the origin 

 of its first branch, the epicritic loss is considerably in excess 

 of the loss of protopathic and deep sensibility. The fact that 

 the fibres conveying deep sensibility pass with the motor 

 branches and run along the tendons is of great importance, 

 because it explains why there is no loss of deep sensibility 

 when a nerve is cut distal t6 the origin of its motor branches. 

 In these cases, a superficial examination may fail to detect the 

 existing sensory loss,, and it is, therefore, necessary to examine 

 for epicritic sensibility in all cases of suspected or possible 

 nerve injury. It must also be observed that the division of 

 tendons, with or without the division of a nerve, will usually 

 lead to some impairment of deep sensibility. 



THE SPINAL NERVES 



Each spinal nerve is formed by the union of an 

 anterior and a posterior nerve-root. The anterior nerve-roots 

 are purely motor, and they arise from the large nerve-cells 

 in the anterior column of grey matter in the spinal medulla. 



