CHAPTER I. 

 THE TISSUES AND MECHANISMS OF DIGESTION. 



174. THE food in passing along the alimentary canal is 

 subjected to the action of certain juices supplied by the secretory 

 activity of the epithelium 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 mingling with the 

 food, produce in it such changes, that from being largely insoluble 

 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 nitrogenous 

 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 sub- 

 stances, differ considerably from proteids in composition and 

 especially in their behaviour in the body ; they are not of sufficient 

 importance to form a class by themselves. 



