316 THE HUMAN BODY. 



Hunger and Thirst, These sensations regulate the tak- 

 ing of food. Local conditions play a part in their produc- 

 tion, but general states of the body are also concerned. 



Hunger in its first stages is due to a condition of the 

 gastric mucous membrane which comes on when the stom- 

 ach has been empty some time; it may then be tempo- 

 rarily stilled by filling the stomach with indigestible sub- 

 stances. But soon the feeling comes back intensified and 

 can only be allayed by the ingestion of nutritive materials; 

 provided these are absorbed and reach the blood their mode 

 of entry is unessential; hunger may be stayed by injections 

 of food into the intestine as completely as by filling the 

 stomach with it. 



Similarly, thirst may be temporarily relieved by moisten- 

 ing the throat without swallowing, but then soon returns ; 

 while it may be permanently relieved by water injections 

 into the veins, without wetting the throat at all. 



Both sensations depend in part on local conditions 

 of sensory nerves, but may be more powerfully excited 

 by poverty of the blood in foods or water ; this deficiency 

 directly stimulates the hunger and thirst centres of the 

 brain. 



The Special Senses are commonly described as five in 

 number, but there are at least six; namely, sight, hearing, 

 touch, the temperature sense, smell, and taste. 



To what is the first stage of hunger due? How may it be tempo- 

 rarily stayed? Need food enter the stomach in order to alleviate 

 hunger? 



How may thirst be temporarily relieved? How permanently 

 without swallowing water? On what do the sensations of hunger 

 and thirst in part depend? How may they be more powerfully ex- 

 cited? Enumerate the special senses. 



