184 PRINCIPLES OF PHYSIOLOGY 



pain, but movement of a muscle or a group of 

 muscles, possibly in some distant part of the 

 body. This is known as a reflex action. It 

 implies a sensory nerve, by which an impulse 

 is carried to a centre, a centre in which a 

 change occurs, the nature of which we do not 

 know, and a motor nerve carrying an impulse 

 to muscles and causing a contraction. The 

 term reflex, although in general use, is mislead- 

 ing, as it suggests something reflected like a 

 ray of light by a mirror ; but we have no 

 better word at present. We know that 

 something occurs in the centre, as time is 

 occupied. Assuming the velocity of the 

 nervous impulse and a given length, both of 

 (a) sensory and of (b) motor nerve, more time 

 is occupied in a reflex action than the sum of 

 times occupied in a and b. This increased 

 time is in c, the centre. Reflex mechanisms 

 play an important part in the body Many 

 are very complicated, involving several sensory 

 and several motor nerves, and even the 

 centres may be complicated. As an example 

 of a simplex reflex, we may take the move- 

 ment of winking of the eyelids. Here the 



