NERVE 131 



3. The inward course of the fibres from the receptors of 

 the body to their termination in different parts of the brain 

 (p. 106 et seq.). 



4. The mode of action of these various parts of the 

 brain (p. 116 et seq.). 



5. The adjustment of the head and body brought about 

 through the vestibulo-cerebellar arrangement (p. 121). 



We must next consider a series of receptors developed in 

 the head which are stimulated by changes at a distance, and 

 which thus warn tlie animal, so that it may prepare to adapt 

 itself to these conditions. 



I. For Chemical Substance. 



1. BUCCAL MECHANISM. 



Taste. 



This is really a modification of cutaneous sensibility, and 

 it might have been studied along with it ; but its close 

 association with the sense of smell makes it more convenient 

 to deal with it here. 



The mouth is richly supplied with receptors for tactile, 

 painful and thermal stimuli, and the receptors for thermal 

 stimuli extend down to the lower end of the crullet. 



In the mouth special receptors have been developed with 

 the object of determining whether any particular material 

 should be swallowed or rejected, according to whether it is 

 beneficial or nocuous. Pavlov found that, in dogs, the flow of 

 saliva varies with the material put in the mouth. Flesh 

 calls forth a flow of viscous saliva which lubricates the 

 mass and facilitates the act of swallowing, while sand placed 

 in the mouth causes a free flow of very w^atery saliva to 

 wash • out the nocuous substance. Pavlov used the reflex 

 flow of saliva to study what he has called Conditioned Reflexes 

 — i.e. reflexes associated with changes in consciousness. 

 Two different notes were sounded near a dog, and it was 

 habitually fed after one of these but not after the other. 

 It was found that sounding the former produced a flow of 

 saliva. 



