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CHAPTER VII. 



Digestion. 



ALL the substances received as food into the stomach, 

 whatever be their nature, must necessarily undergo many 

 changes of chemical composition before they can gain ad- 

 mission into the general mass of circulating fluids; but the 

 extent of the change required for that purpose will, of course, 

 be in proportion to the difference between the qualities of 

 the nutritive materials in their original, and in their assimi- 

 lated state. The conversion of vegetable into animal mat- 

 ter necessarily implies a considerable modification of proper- 

 ties; but even animal substances, however similar may be 

 their composition to the body which they are to nourish, 

 must still pass through certain processes of decomposition, 

 and subsequent recombination, before they can be brought 

 into the exact chemical state in which they are adapted to 

 the purposes of the living system. 



The preparatory changes we have lately been occupied in 

 considering, consist chiefly in the reduction of the food to a 

 soft consistence, which is accomplished by destroying the co- 

 hesion of its parts, and mixing them uniformly with the fluid 

 secretions of the mouth; effects which may be considered as 

 wholly of a mechanical nature. The first real changes in its 

 chemical state are produced in the stomach, where it is con- 

 verted into a substance termed Chyme; and the process by 

 which this first step in the assimilation of the food is pro- 

 duced, constitutes what is properly termed Digestion. 



Nothing has been discovered in the anatomical structure 

 of the stomach, tending to throw any lighten the means by 

 which this remarkable chemical change is induced on the 



