672 PHYSIOLOGICAL CHEMISTRY. 



56. DIGESTION. 



General remarks. It has been stated that foods are divided into 

 two classes, inorganic and organic, and that the fetter are subdivided 

 into proteins, carbohydrates, and fats ; and also that the term diges- 

 tion refers to the process by which organic foods are altered in such 

 a manner that they may be absorbed. 



The process of digestion is both mechanical and chemical. By 

 the mechanical part of the process the food-material is disintegrated, 

 propelled along the alimentary canal, and mixed with the different 

 digestive secretions. These latter cause a chemical change, usually 

 hydrolysis, of the food, converting it into soluble and easily absorb- 

 able substances. For convenience of study the process is divided 

 into salivary, gastric, and intestinal digestion ; and the secretions 

 and chief alterations of the nutrients in these portions of the tract are 

 considered separately. It should be remembered, however, that these 

 three processes are closely interdependent, and any disturbance of 

 function, mechanical or chemical, in one part of the digestion will 

 disturb and derange all. 



Salivary digestion. The first part of the process of digestion is 

 accomplished in the mouth, and consists in the breaking up of the 

 food by the teeth and mixing it with saliva, the process being known 

 as mastication. In addition, the saliva, to a limited extent, converts 

 starch into maltose. This action of the saliva is due to its ferment 

 ptyalin. Other functions of the saliva are to keep the mucous mem- 

 brane of the mouth moist and to lubricate the food bolus. 



Saliva is the mixed secretion of the parotid, submaxillary, sub- 

 lingual, and buccal glands. The quantity secreted in a day varies 

 from 600 to 1500 c.c. The flow is easily excited by reflex stimula- 

 tion, as by the smell or sight of food, or by chewing of some insolu- 

 ble substance. Saliva appears as a viscid, frothy, tasteless, inodor- 

 ous liquid, of a sp. gr. of 1.002 to 1.008. The reaction to litmus is 

 generally slightly alkaline, but may become acid under pathological 

 conditions. Saliva as it appears in the mouth contains food particles 

 and numerous micro-organisms. The average composition of saliva 

 is as follows : 



Water 99.49 



Mucin and epithelium 0.13 



Fatty matter 0.11 



Ptyalin, maltase, and other organic matter . . . .0.12 

 Salts 0.15 



The salts are alkali and earthy phosphates, carbonates and chlo- 

 rides, and potassium sulphocyanate. The latter occurs in variable 



