CHAPTER XXIII. 



DIGESTION. 



The Object of Digestion. Of the various foodstuff 15 

 swallowed, some are already in solution and ready to dialyze 

 at once into the lymphatics and blood-vessels of the alimen- 

 tary canal; others, such as a lump of sugar, though not 

 dissolved when put into the mouth, are readily soluble in 

 the liquids found in the alimentary canal, and need no fur- 

 ther digestion. In the case of many most important food- 

 stuffs, however, special chemical changes have to be wrought, 

 either with the object of converting insoluble bodies into 

 soluble, or non-dialyzable into dialyzable, or both. The 

 different secretions poured into the alimentary tube act in 

 Yarious ways upon different foodstuffs, and at last get them 

 into a state in which they can pass into the circulating 

 medium and be carried to all parts of the Body. 



The Saliva. The first solvent that the food meets with 

 is the saliva, which, as found in the mouth, is a mixture of 

 pure saliva, formed in parotid, submaxillary, and sublingual 

 glands, with the mucus secreted by small glands of the oral 

 mucous membrane. This mixed saliva is a colorless, cloudy, 

 feebly alkaline liquid, " ropy" from the mucin present in 

 it, and usually containing air-bubbles. Pure saliva, as ob- 

 tained by putting a fine tube in the duct of one of the sali- 

 vary glands, is less tenacious and contains no imprisoned 

 air. 



The uses of the saliva are for the most part physical and 

 mechanical. It keeps the mouth moist and allows us to 

 speak with comfort; most young orators know the distress 

 occasioned by the suppression of the salivary secretion 

 through nervousness, and the imperfect efficacy under such 



