J40 LIFE : OUTLINES OF GENERAL BIOLOGY 



effect to the motor orders. This hnkage is prearranged for us before 

 birth. It is part of our inheritance, and requires no learning or 

 p:)ractice. What occurs is called by th.c physiologists an "uncondi- 

 tioned reflex"; and it is one of the commonest occurrences, both in 

 man and beast. When we shut our eye at the approach of a missile; 

 when we swallow what touches the back of our mouth; when we 

 sneeze after a snuff; when we cough up a crumb that threatens to 

 go down the wTong way; when we draw back our fingers from a hot 

 cinder we are illustrating various forms of unconditioned reflex, 

 some more complicated than others. 



When a sea-anemone closes its tentacles on a piece of food, 

 when a starfish breaks off a pinned-down arm, when a crab breaks 

 off a badly damaged leg, when an earthworm jerks itself into its 

 hole if the ground vibrates under the blackbird's foot, when a snail 

 draws in its horn at a touch, when a plaice puts on the colour 

 of the sand on which it has come to rest, when a nestling opens 

 its mouth at the touch of food in its mother's bill, we have 

 to do with unconditioned reflexes; and the animal world is full 

 of them. 



It is characteristic of unconditioned reflexes that they are inborn, 

 that they do not require to be learned, that they are shared by all 

 the members of the species, and that they are often quite inde- 

 pendent of the brain for their performance. It should be noted, 

 however, that even in the case of a very simple unconditioned 

 reflex like the knee-jerk we started with, a message goes to the brain 

 to report the occurrence. Moreover, by an effort of will, if we are 

 forewarned, we can prevent the straightening of the leg when the 

 knock comes. The brain is sometimes able to suppress the normal 

 reflex. 



This suppression of a reflex used to be utilised in Oriental ordeals. 

 Everyone knows that the presence of food in the mouth normally 

 I)rompts a flow of saliva, which makes swallowing easier. But some- 

 times, under the influence of great sorrow, or fear, or some other 

 emotion, we are unable to swallow, partly because the food remains 

 so dry. Thus the sus|x;cted criminal in the East was given rice to 

 chew, the somewhat ingenuous theory being that if he could salivate 

 readily he must be innocent, whereas if he could not moisten the 

 rice and swallow it he must be guilty. 



A very valuable book, not easy reading, by the distinguished 

 Russian physiologist, I. P. Pavlov, is devoted to "Conditioned 

 Reflexes", which play an important role in human and animal life. 

 Let us flrst of all explain how they differ from "Unconditioned 

 Reflexes". 



If a dog is shown a piece of flesh its mouth waters. If a whistle is 

 always sounded when the flesh is shown to the dog an a.ssociation 

 is gradually formed between the sound and the prospect of a meal. 



