1 66 PHYSIOLOGY OF NERVE. [OH. XV. 



These, however, are instances of reflex action which are 

 accompanied with conscious sensation, but like all pure reflex 

 actions are not under the control of the will. 



An instance of a reflex action not accompanied with conscious- 

 ness is seen in a man with his spinal cord cut across or crushed, 

 so that any communication between his brain and his legs is 

 impossible. He cannot move his legs voluntarily and is un- 

 conscious of any feelings in them. Yet when the soles of his 

 feet are tickled he draws his legs up, the centre of reflex 

 action being in the grey matter of the lower region of the 

 spinal cord. 



For a reflex action, three things are necessary : (i) an afferent 

 nerve, (2) a nerve-centre consisting of nerve-cells to receive the 

 afferent impulse and send out an efferent impulse, and (3) an 

 efferent nerve along which the efferent impulse may travel. If 

 the reflex action is a movement, the afferent nerve is called 

 excito-motor ; if it is a secretion, the afferent nerve is called excito- 

 secretory, and similarly afferent nerves may also be excito-accelerator, 

 excito-inhibitory, etc. 



3. Intercentral nerves are those which connect nerve-centres 

 together ; they connect different parts of brain, and of the cord 

 to one another, and we shall find in our study of the nerve-centres 

 that they are complex in their arrangement. 



Investigation of the Functions of a Nerve. 



There are always two main experiments by which the function 

 of a nerve may be ascertained. The first is section, the second is 

 stimulation. 



Section consists in cutting the nerve and observing the loss of 

 function that ensues. Thus, if a motor nerve is cut, motion of 

 the muscles it supplies can no longer be produced by activity 

 of the nerve-centre ; the muscle is paralysed. If a sensory nerve 

 is cut, the result is loss of sensation in the part it comes from. 



Stimulation of the cut nerve is the opposite experiment. 

 When a nerve is cut across, one piece of it is still connected 

 with the brain or spinal cord ; this is called the central end ; the 

 other piece, called the peripheral end, is still connected with some 

 peripheral part of the body. Both the central and the peripheral 

 end should be stimulated ; this is usually done by means of 

 induction shocks. In the case of a motor nerve, stimulation of 

 the central end produces no result ; stimulation of the peripheral 

 end produces a nervous impulse which excites the muscles to con- 

 tract. In the case of a sensory nerve, stimulation of the peripheral 



