246 THE TISSUES AND MECHANISMS OF DIGESTION. 



4. Saline or mineral bodies, and water. These salts are for the most part 

 inorganic salts, and this class differs from the three preceding classes inas- 

 much as the usefulness of its members to the body lies not so much in the 

 amount of energy which may be given put 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 the 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 characterized by the absorp- 

 tion of fluid from the interior of the alimentary 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 various 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 manu- 

 facture 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 muscular 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. 



THE CHARACTERS AND PROPERTIES OF SALIVA AND GASTRIC JUICE. 



Saliva. 



183. Mixed saliva, as it appears in the mouth, is a thick, glairy, gen- 

 erally frothy and turbid fluid. Under the microscope it is seen to contain, 

 beside the molecular debris of food, bacteria and other organisms (fre- 

 quently cryptogamic spores), epithelium scales, mucous corpuscles and 

 granules, and the so-called salivary corpuscles. Its reaction in a healthy 

 subject is alkaline, especially when the secretion is abundant. When the 

 saliva is scanty, or when the subject suffers from dyspepsia, the reaction of 

 the mouth may be acid. Saliva contains but little solid matter, on an aver- 

 age probably about 5 per cent., the specific gravity varying from 1002 to 

 1006. Of these solids, rather less than half, about 2 per cent., are salts 

 (including at times a minute quantity of potassium sulphocyanate). The 

 organic bodies which can be recognized in it are globulin and serum albumin 

 (see 16, 17), found in small quantities only other obscure bodies occur- 

 ring in minute quantity, and mucin ; the latter is by far the most conspicu- 

 ous organic constituent, the glairiness or ropiness of mixed and other kinds 

 of saliva being due to its presence. 



Mucin. If acetic acid be cautiously added to mixed saliva, the viscidity 

 of the saliva is increased, and on further addition of the acid a semi-opaque 

 ropy mass separates out, leaving the rest of the saliva limpid. This ropy 



