The Physiological Basis of Thirst. 



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however, attention has been paid to the bodily need which accompanies it ; 

 apparently, since the need is a general one, the sensation has been supposed 

 to be general, and the thirst which everybody experiences and knows about 

 has been classed as an associated secondary phenomenon or the peripheral 

 reference of a central change. The really doubtful feature in this view of 

 thirst, just as in the older conception of hunger, is the " general sensation." 

 That even the early stages of a need of water may be accompanied by 

 increased irritability, and a vague sense of weakness and limpness, is not 

 denied. But the thirsty man does not complain of these general conditions. 

 He is tormented by a parched and burning throat, and any explanation of the 

 physiological mechanism for maintaining the water content of the body must 

 take into account this prominent fact. 



In looking for a mechanism which would automatically keep up the water 

 supply of our bodily economy, we may follow two clues ; first, that there may 

 be a peripheral arrangement which in the presence of a general bodily need for 

 water would lead to dryness of the mouth and throat ; and second, that a 

 peripheral arrangement of this nature should be especially characteristic of 

 animals which are constantly and rapidly losing water and require repeated 

 renewal of the supply. These two clues offer a biological approach to the 

 explanation of thirst which I wish to utilise. 



In one sense all animals are constantly losing water, for even in the 

 simplest forms waste material is excreted in solution. With respect to water 

 loss, however, we should expect to find a marked difference between animals 

 living in water itself and those living in air. Indeed, it is difficult to conceive 

 of an animal living in water as experiencing thirst. The entire body surface 

 and the mouth and throat are throughout active life continuously bathed in a 

 moving flow. The food is taken wet from a wet medium. Probably renal 

 activity and the secretion of the digestive glands are the only important ways 

 for water to leave the economy ; and the digestive secretions are soon largely 

 re-absorbed. In contrast, the land animals, mammals, for example, lose 

 moisture not only in these ways but also by the moistening of dry food, by 

 evaporation from the extensive surface of the lungs, and by the action of 

 innumerable sweat glands. It is because of the possibility of great and rapid 

 loss of water from its body that the land animal has special need for an 

 assurance of adequate supply. 



In the water inhabitant the skin, and the mouth and gullet, are all kept 

 wet by the medium in which he lives and moves. In the process of evolution, 

 however, as organisms changed their habitat from water to air, the skin 

 became dry and scaly. Of the parts which in marine animals were constantly 

 bathed by water, only tlie mouth and throat continue to be moist. These 



