292 DIGESTION. 



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time by moistening the dry fauces ; but may be relieved 

 completely by the introduction of liquids into the blood, 

 either through the stomach, or by injections into the 

 blood-vessels, or by absorption from the surface of the 

 skin or the intestines. The sensation of thirst is per- 

 ceived most naturally whenever there is a disproportion- 

 ately small quantity of water in the blood : as well, 

 therefore, when water has been abstracted from the blood, 

 as when saline or any solid matters have been abundantly 

 added to it. We can express the fact (even if it be not 

 an explanation of it), by saying that the nerves of the 

 mouth and fauces, through which the sense of thirst is- 

 chiefly derived, are more sensitive to this condition of the 

 blood than other nerves are. And the cases of hunger 

 and thirst are not the only ones in which the mind derives, 

 from certain organs, a peculiar predominant sensation of 

 some condition affecting the whole body. Thus, the sensa- 

 tion of the " necessity of breathing," is referred especially 

 to the lungs; but, as Volkmann's experiments show, it 

 depends on the condition of the blood, which circulates 

 everywhere, and is felt even after the lungs of animals 

 are removed ; for they continue, even then, to gasp and 

 manifest the sensation of want of breath. And, as with 

 respiration when the lungs are removed, the mind may 

 still feel the body's want of breath ; so in hunger and 

 thirst, even when the stomach has been filled with innu- 

 tritious substances, or the pneumogastric nerves have 

 been divided, and the mouth and fauces are kept moist, 

 the mind is still aware, by the more obscure sensations in 

 other parts, of the whole body's need of food and 

 water. 



The influence of the nervous system on the secretion of gastric 

 fluid, is shown plainly enough in the influence of the mind 

 upon digestion in the stomach ; and is, in this regard, well 

 illustrated by several of Dr. Beaumont's observations. 

 M. Bernard also, watching the act of gastric digestion in 



