CHAPTER II. 



(a) CHEMICAL CONSTITUENTS OF BODY AND FOOD. 

 (b) ALIMENTARY SUBSTANCES. 



DIGESTION has been described as the physical and chemical alter- 

 ation of the foodstuffs into forms better fitted for absorption by the 

 action of certain soluble ferments, the digestive enzymes. 



The animal organism had its birth in a single ovum or cell, which, 

 under certain favoring circumstances and conditions, developed into 

 a mass of simple cells. As development proceeded, this aggregation 

 became differentiated into tissues, by the grouping of the cells, altered 

 by chemical changes in the substance of the cells themselves, by 

 alterations in their shapes, and by deposits of intercellular substances. 

 As the organism continued to grow, the various parts became more 

 and more complex by use and development until it presented a 

 highly complex unit. 



In the metabolism of the cell it was learned that the various cells 

 while performing their various vital phenomena must constantly 

 maintain a very nice balance in respect to waste and repair. That is, 

 the various kinds of cells took out from their environments those sub- 

 stances that were necessary for their economy to build themselves up 

 and grow, while the waste-products were excreted. A distinctive 

 property of the cells was the selective power exercised in regard to 

 different nutrient materials with which they came into contact. Al- 

 though the surrounding media might contain many kinds of food, yet 

 cells of a particular kind took only that for themselves which was best 

 adapted to their wants, disregarding entirely all the others. As 

 there was a great variety of cells, there must necessarily be a cor- 

 responding variety of foodstuffs. 



What is true of the cells is true of that of which they are but 

 components or units: the body. Among the phenomena produced 

 by the waste of the solid constituents of the body and the loss of the 

 fluid or watery parts of the tissues are the sensations of hunger and 

 thirst. These sensations of appetite excite the desire to take food, 

 which by the processes of digestion is prepared for absorption and cir- 

 culation in the blood, to supply the various needs of the organism. 



The term food includes all those substances received into the 

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