38 PHYSIOLOGY. 



Gelatin is the substance produced by heating in dilute acetic acid 

 for several days, the collagen of connective-tissue fibers. It possesses 

 the property of setting into a jelly when its concentration is greater 

 than 1 per cent. When digested it is converted into a peptone, and, 

 although readily absorbed, is not able to take the place of a true 

 proteid, since it cannot build up nitrogenous tissue, being valuable 

 only as a means of storing up energy. 



Keratin is the horny material forming the outer layer of the 

 epidermis, hair, wool, nails, hoofs, etc. 



Elastin of elastic tissue belongs to this group. 



7. Histories. 



To the histones belong globin, the proteid which is separated 

 from hsemaglobin by decomposing acids and alkalies. 



8. Protamins. 



The protamins are salmin, clupein, scombrin, sturin, etc. 



ALIMENTARY SUBSTANCES. 



We have learned that the body is composed of the chemical con- 

 stituents or proximate principles, carbohydrates, fats, and proteids 

 comprising the organic group, and water and salts the inorganic class. 

 In order that the nutrition of the body may proceed normally, it is 

 very apparent that those principles must be supplied in the food, in 

 the proper proportions and quantities. So, a proper diet for man is 

 one containing the proximate principles in their proper proportions, 

 the value of it depending mainly on the amount of carbon and nitro- 

 gen present. 



The elements, as elements, are not valuable; it is only when 

 they are in combination that they serve their proper ends as foods. 

 For the elements, to constitute an organic product, must be united 

 previously by some living organism. It is not often that the alimen- 

 tary substances are used by us as Nature furnishes them, even though 

 they contain the proper ingredients. One requisite is that they 

 should be presented in a digestible form. Water, heat, and condi- 

 ments are the three agents used to make food more palatable and 

 digestible. Water helps to soften the insoluble substances, and 

 to dissolve the principal substances. Heat modifies the foods 

 still more, so that they acquire different characters. Condiments 

 give physical satisfaction and enjoyment, and, at the same time, they 

 please the taste. 



