514 DIGESTION 



the hunger contractions of the stomach. No doubt it stimulates the 

 appetite, but that, as we have seen, is a psychic affair. 



Thirst 



The Sensation of Thirst has been believed by some observers to be a 

 general one either dependent primarily upon dehydration of the tissues 

 of the body, including those of the central nervous system, or resulting 

 secondarily from a rise in the osmotic pressure of the blood the hyper- 

 tonicity producing disturbances in the nerve cells. It is probable, how- 

 ever, that the thirst sensation is a purely local one, namely, drying of the 

 mucous membranes of the mouth and pharynx. This view is supported 

 by the fact, that if the pharynx of a person suffering from thirst be 

 painted with a solution of cocaine all craving is relieved and it does not 

 return until the effect of the anesthetic has passed off. That thirst 

 may be produced through the drying of the interior of the mouth as by 

 prolonged speaking, or mouth breathing, is common experience. Admin- 

 istration of atropine acts similarly by inhibiting the oral secretions. 

 According to Cannon 21 the salivary glands play the chief role in the 

 mechanism for the production of the thirst sensations. He finds that the 

 secretion of these glands, which normally moistens the interior of the 

 mouth and pharynx, is reduced markedly (60 to 75 per cent) after a 

 few hours of abstinence from fluids. Since the composition of the sali- 

 vary secretion is from 97 to 99 per cent water it is readily seen how 

 a fall in the general water content of the body would bring about a re- 

 duction of salivary flow with consequent drying of the oral and pharyn- 

 geal mucous membranes. This view ascribes to the salivary glands the 

 important duty of keeping sentinel over the water content of the body, 

 the sensation of thirst being the signal which warns the organism that 

 the tissue fluids require to be replenished. 



