SECTION II. 



FUNCTIONS OF NUTRITION. 



CHAPTER I. 

 DIGESTION. 



THE first act in the process of nutrition is that by which the food is 

 liquefied and made capable of absorption. Animals and man require 

 for their sustenance organic materials ; that is, substances which have 

 already formed part of organized bodies. When taken as food these 

 matters are almost invariably solid or semi-solid, and insoluble in water. 

 The alimentary constituents of meat, grain, herbage, and vegetables are 

 mainly solid in form ; and even the nutritious substances, naturally fluid, 

 such as milk, white of egg, and other albuminous liquids, are usually 

 more or less solidified by cooking, when used for human food. These 

 substances, accordingly, before they can be taken up by the blood- 

 vessels, and made available for the nourishment of the tissues, need to 

 be reduced to a soluble condition. The preliminary act, by which this 

 is accomplished, is the process of digestion. 



While there are many variations of detail in the digestive process, 

 according to the structure and habits of different animals, its essential 

 features are everywhere the same. The food is taken into a canal, 

 running through the body from mouth to anus, known as the " ali- 

 mentary canal," exhibiting, at various points, enlargements, con- 

 strictions, or diverticula, and receiving the secretions of various 

 accessory glands. While passing through this canal it comes in 

 contact with certain digestive fluids, secreted by the mucous membrane 

 of the canal and by the accessory glands, which act tipon it in such a 

 way as to liquefy its ingredients, or otherwise modify their physical 

 condition. As the alimentary mass passes from above downward, 

 urged by the muscular action of the intestine, its liquefied parts are 

 removed by absorption ; while the remainder, consisting of the indi- 

 gestible portions, with the refuse of the intestinal secretions, gradually 

 acquires the consistency of feces, and is finally discharged under that 

 form from the intestine. 



The alimentary canal varies in different animals, according to the 

 comparative development of its different parts. In herbivorous 

 animals generally, it is longer and more complicated than in the 

 carnivora. In man, where it holds an intermediate position in this 



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