238 UNCONSCIOUS NERVOUS OPERATIONS 



constantly cause pressure upon the tissues and so force 

 the lymph onward in the vessels, the valves preventing 

 any return. 



344. Assimilation. Though the food has been masti- 

 cated, digested, and absorbed, it has not yet nourished 

 the body. Still another process is needful before the 

 new material becomes part of the continually wasting 

 tissues. That process is called assimilation, and, though 

 we cannot pretend to understand it, it may be described 

 as the action of the living cells in choosing, appropriat- 

 ing, and building into their own substance the suitable 

 elements in the food-laden fluid which comes to them 

 from the alimentary canal and from the lungs. 



Correlative to the process of assimilation is the destruc- 

 tive process by which the cells, by combustion and other 

 chemical changes, break up and send out as waste the 

 substances of their structure, to be expelled from the 

 body as excretions. 



345. Hunger and Thirst. We associate our feelings of 

 thirst with a dry ness of the mucous membrane of the 

 mouth and throat, and we say our throats are " parched " 

 when we are very thirsty. But under ordinary circum- 

 stances the feeling of thirst arises from a general con- 

 dition of the system, in which the throat shares, due to 

 a lack of water in the blood, or rather in the lymph. 

 Thirst may be temporarily relieved by moistening the 

 mucous membrane of the soft palate. Hence follows the 

 inference that the afferent nervous impulses originate 

 there, and are caused by a too great removal of water 

 from the lymph of the investing membrane. 



Hunger is referred in our consciousness to the par- 

 ticular locality of the stomach, and that organ seems to 

 us to be empty when we are hungry. Indigestible mate- 



