312 FOOD-STUFFS. [Book ii. 



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 

 (§ 66) ; they all possess, in view of the oxidation of both their 

 carbon and their hydrogen, a large amount of potential energy. 



3. Carbo-hydrates, 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 oxidised give out, weight for weight, 

 less heat than do fats. 



4. Saline or Mineral Bodies, and Water. These salts aie for 

 the most part inorganic salts ; and this class differs from the three 

 preceding classes inasmuch as the usefulness of its members to 

 the body lies not so much in the amount of energy which may 

 be given out by their oxidation, as in the various influences which, 

 by their presence, they exercise on the metabolic events of the 

 body. 



These several food-stuffs are variously acted upon in the 

 several parts of the alimentary canal, and we may distinguish, as 

 the food passes along the digestive tract, three main stages : 

 digestion in the mouth and stomach, digestion in the small 

 intestine, and digestion in the large intestine. In many animals 

 the first stage is, to a large extent, preparatory only to the second 

 which in all animals is the stage in which the food undergoes the 

 greatest change ; in the third stage the changes begun in the 

 previous stages are completed, and this stage is especially ehaiac- 

 terised by the absorption of fluid from the interior of the alimen- 

 tary canal. 



It will be convenient to study these stages, more or less apart, 

 though not wholly so, and it will also be convenient to consider 

 the whole subject of digestion under the following heads: — 



First, the characters and properties of the vaiious juices, and 

 the changes which they bring about in the food eaten. 



Secondly, the nature of the processes by means of which 

 the epithelium cells of the various glands and various tracts of 

 the canal are able to manufacture so many various juices out of the 

 common source, the blood, and the manner in which the secretory 

 activity of the cells is regulated and subjected to the needs of the 

 economy. 



Thirdly, the mechanisms, here as elsewhere chiefly of a mus- 

 cular nature, by which the food is passed along the canal, and 

 most efficiently brought into contact with the several juices. 



Fourthly and lastly, the means by which the nutritious digested 

 material is separated from the undigested or excremental material, 

 and absorbed into the blood. 



