124 Inside the Living Cell 



when the crab and plate are presented, it merely 'leans forward from 

 its home and watches the situation'.^ 



The most famous experiments were, however, those of Pavlov. If a 

 hungry dog is shown food, its mouth begins to water. It is obvious 

 that the sensation given by the sight and smell of the food in some 

 way causes the salivary glands to excrete. This is a simple reflex 

 action, and the salivation is an involuntary response. If a bell is rung 

 before the food is shown, the animal will associate the sound of the 

 bell with food and the sound of the bell will, for a time, cause his 

 mouth to water. This is a conditioned reflex. It is obvious that connec- 

 tions have been established in the brain between messages arriving 

 from the eyes or nose and the ear, in such a way that a message 

 received by the ear stimulates a response which is normally produced 

 by ear and nose messages. 



The conditioned reflex was regarded by Dr J. B. Watson and the 

 behaviourist school of psychology as a sufficient explanation of 

 nearly the whole of animal behaviour. They regarded all responses 

 of the individual animal to environment as either direct (like the knee 

 jerk) or somewhat modified by recoflections of previous situations 

 which might cause the simplest response to be delayed or inhibited; 

 or 'conditioned' by circumstances indirectly connected with them 

 (like the sound of Pavlov's bell). 



But in fact the 'conditioned reflex' is itself a very complicated 

 phenomenon and the existence of many of the profounder abilities 

 of the brain are implied in it. As I said, it obviously requires the 

 existence of memory of past experiences and the abiUty to interpret 

 present experiences in the light of past, and therefore the ability to 

 learn from experience. 



These must be discussed in detail; but in the first place it will be 

 helpful to have a clearer idea of the actual organization in the brain, 

 and some notion of how it handles the messages it receives from the 

 senses. 



BRAIN ACTIVITY 



The brain of a large animal, as I have said, consists of an enormous 

 mass of neurones. Some distinct parts can be demonstrated; the cere- 

 bellum, which is largely concerned with the more 'animal' functions 

 such as balance and the posture of the body, and the cerebrum, 

 which is covered by the cerebral cortex, which reaches its greatest 

 development in man, a great convoluted membrane, about a hundred 



^ B. B. Boycott and J, Z. Young. S.E.B. Symposium on Physiological 

 Mechanisms of Animal Behaviour (1950), p. 432. 



