NUTRITIVE FUNCTIONS DIGESTION. 281 



which it contains. The gastric fluid is found to contain 

 more acid, in proportion as the food is more difficult of 

 digestion. It is only secreted, when food is present in the 

 stomach. 



18. The aliment then is converted into chyme, chiefly by 

 the gastric fluid ; aided, however, by the motions of the sto- 

 mach. This is proved by the fact, that this fluid will dis- 

 solve alimentary substances out of the stomach : the chyme 

 which is prepared by artificial digestion, presenting the same 

 sensible properties as that which is found in the stomach. 

 Such is the power of the gastric fluid, that it will dissolve 

 bones, not only in the stomach, but out of it. It curdles 

 milk ; and for this purpose the rennet, or stomach of the 

 calf, is used by farmers in making cheese. But every thing 

 which is coagulated in the stomach, is dissolved again, in its 

 conversion into chyme. 



19. But digestion is a vital and not a chemical process. 

 Though aliment may be reduced to a substance resembling 

 chyme, by the action of gastric fluid out of the body ; yet 

 it is destitute of all those peculiar properties which assimi- 

 late it to the nature of a living animal. The food may 

 doubtless be brought to a fluid state by a chemical process, 

 and even alimentary principles may in the same way be 

 changed into each other, as starch into sugar and gum ; but 

 there is still another power, which may be called vitalizing 

 or organization, by which alimentary substances are brought 

 into such a condition as adapts them for an intimate union 

 with the living body. Such a power is beyond the reach of 

 chemical action. 



20. The time which is required to change food into chyme, 

 varies according to the nature of the food. Animal food is 

 digested sooner than vegetable. The average time required, 

 is about three hours and a half. A good deal depends, also, 

 on the degree of mastication it has undergone. If swallowed 

 in large masses its solution must go on slowly. 



21. As fast as the aliment is changed into chyme, it pass- 



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