634 LIFE : OUTLINES OF GENERAL BIOLOGY 



Another example of an unconditioned reflex is the secretion of 

 saliva which follows the introduction of food into the mouth; this 

 is a reflex not easily suppressed by voluntary effort, perhaps, but 

 suppressed by the greater claims of a stronger stimulus, such as 

 sudden pain, or by emotional disturbance, such as iean; which is the 

 physiological basis of the ordeal by chewing rice, used for the 

 conviction of malefactors in the East. But what are we to say of 

 the secretion of saliva which may take place when we merely smell 

 a savoury dish, or see someone eating a lemon, or — subtler still — 

 when we hear the gong sounded for dinner ? These are cases of what 

 Pavlov calls "conditioned" reflexes. They are not conmion to all 

 members of the species, for a man who has never tasted lemon can 

 whistle unconcernedly while his neighbour sucks one; they are not 

 inborn, for the sound of the gong means nothing to the child. They 

 are, none the less, reflexes, and behave somewhat like simpler 

 reflexes in their relation to the higher activities of the brain; but 

 they are learned or acquired by each individual in his own experience. 

 We may note that such conditioned reflexes are always based upon 

 inborn reflexes; the response is the old one, though the stimulus is 

 subtler than before — the aroma or the mere sight or mention or 

 thought of food instead of its physical presence in contact with the 

 receptor organs of the mouth. 



This salivary reflex is the one which Prof. Pavlov and his 

 colleagues have found most useful, since it is possible to estimate 

 the efficiency of .the reflex by measuring the amount of saliva 

 secreted. Within a specially-built laboratory at Petrograd, in which 

 disturbing factors were reduced to a minimum, they taught dogs to 

 associate the giving of food with some other accompanying stimulus 

 — a sound, a flash of light, an odour, or a touch — until a conditioned 

 reflex was established and the "irrelevant" stimulus alone was able 

 to cause a secretion of saliva. It would lead us too far to consider 

 the experiments in detail, or the light which they cast on the pro- 

 cesses of learning and unlearning, on the behaviour of animals 

 generally and the functions of the brain. For these the reader must 

 be referred to Prof. Pavlov's great book, or to such a summary as 

 that given in Prof. Lovatt Evans's Recent Advances in Physiology. 



(IX) REACTIONS IN PROTOZOA. -Descending to the level of 



unicellular organisms we iind a number of established reactions, 

 analogous to reflex actions; but it is desirable to keep that precise 

 term for animals with a linkage of receptor-, motor-, and effector- 

 cells. We are referring now to the fixed reactions that many Protozoa 

 exhibit in response to divers<^ environmental stimuli. Their estabhsh- 

 ment may simply Ix' the outcome of the selection of the relatively 

 fitter responses. Thus the SlipjxT Animalcule (Paramoccium), coming 

 within the sphere of influence of some detrimental stimulus, reverses 



