BOOK II. 



THE TISSUES OF CHEMICAL ACTION, THEIR RESPECTIVE 

 MECHANISMS, NUTRITION. 



CHAPTER I. 



THE TISSUES AND MECHANISMS OF DIGESTION. 



182. THE food in passing along the alimentary canal is subjected to 

 the action of certain juices supplied by the secretory activity of the epithe- 

 lial cells which line the canal itself or which form part of its glandular 

 appendages. These juices, viz., saliva, gastric juice, bile, pancreatic juice, 

 and the secretions of the small and large intestines, poured upon and min- 

 gling with the food produce in it such changes that, from being largely insol- 

 uble, it becomes largely soluble, or otherwise modify it in such a way that 

 the larger part of what is eaten passes into the blood, either directly by 

 means of the capillaries of the alimentary canal, or indirectly by means of 

 the lacteal system, while the smaller part is discharged as excrement. 



Those parts of the food which are thus digested, absorbed, and made use 

 of by the body, are spoken of as food-stuffs (they have also been called 

 alimentary principles'), and may be conveniently divided into four great 

 classes : 



1. Proteids. We have previously ( 15) spoken of the chief characters 

 of this class, and have dealt with several members in treating of blood and 

 muscle. We may here repeat that in general composition they contain in 

 100 parts by weight " in round numbers" rather more than 15 parts of 

 nitrogen, rather more than 50 parts of carbon, about 7 parts of hydrogen, 

 and rather more than 20 parts of oxygen ; though essentially the nitro- 

 genous bodies of food and of the body, they are made up of carbon to the 

 extent of more than half their weight. 



The nitrogenous body gelatin, which occurs largely in animal food, and 

 some other bodies of less importance, while more closely allied to proteid 

 bodies than to any other class of organic substances, differ considerably from 

 proteids in composition and especially in their behavior in the body ; they 

 are not of sufficient importance to form a class by themselves. 



2. Fats, frequently but erroneously called hydrocarbons. These vary 

 very widely in chemical composition, ranging from such a comparatively 

 simple fat as butyrin to the highly complex lecithin ( 69) ; they all possess, 

 in view of the oxidation of both their carbon and their hydrogen, a large 

 amount of potential energy. 



3. Carbohydrates, or sugars and starches. These possess, weight for 

 weight, relatively less potential energy than do fats ; they already contain 

 in themselves a large amount of combined oxygen, and when completely 

 oxidized give out, weight for weight, less heat than do fats. 



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