THE BIOLOGY OF HEALTH 137 



what are called reflexes. Oar finger touches something 

 hot or sharp and we draw it away without knowing or 

 willing. That is a simple reflex action ; what is it 

 that happens ? An external stimulus acting on the nerve- 

 endings in our finger-tips is followed by a message which 

 travels by sensory nerve-fibres up our arm to cells in 

 the ganglia on the dorsal roots of the spinal nerves ; 

 the signal is passed on from these receiving elements 

 into the spinal cord and stimulates internuncial cells ; 

 from these the message is passed on in the cord to the 

 motor nerve-cells that give out orders ; a command is 

 sent via motor nerve-fibres to the muscles, and effective 

 action results, — all in a shorter time than we take to 

 say " Jack Kobinson." 



These reflexes occur independently of our will ; they 

 take place in virtue of certain structural connections 

 which are established in the nervous system in the 

 course of development. A sensitive nerve-ending is 

 linked to a receptive neuron, this to an internuncial 

 cell, this to a motor neuron, and this to a muscle. The 

 linkages are established as part of our inheritance, and 

 though people differ in the rapidity of their reflexes, 

 there is a common stock shared by all mankind. The 

 example we took was a very simple reflex ; every one is 

 familiar with many that are more complex, such as cough- 

 ing, sneezing, sucking, swallowing, shivering. It would 

 take us a long time to learn to sneeze, but the infant 

 sneezes without a lesson. Some biologists would say 

 that its body unconsciously remembers how to sneeze ; 

 what is certain is that an inborn organisation has made 



