CHE3IISTBY OF DIGESTION AND NUTRITION. 283 



B. Salivary Digestion. 



The first of the digestive seeretions with which the food comes in contact 

 is saliva. This liquid is a mixed secretion from the six large salivary glands 

 (parotids, submaxillaries, and sublinguals) and the smaller mucous and serous 

 glands that open into the mouth. The physiological anatomy of these 

 glands and the mechanism by which the secretions are produced and regulated 

 will be found described fully in the section on Secretion ; we are concerned 

 here only with the composition of the secretion after it is formed, and with its 

 action upon foods. 



Properties and Composition of the Mixed Saliva. — Filtered saliva is a 

 clear, viscid, transparent liquid. As obtained usually from the mouth, it is 

 more or less turbid, owing to the presence in it, in suspension, of particles 

 of food or of detached cells from the epithelium of the mouth. A some- 

 what characteristic cell contained in it in small numbers is the so-called 

 " salivary corpuscle." These bodies are probably leucocytes, altered in struc- 

 ture, that have escaped into the secretion. So far as is known, they have no 

 physiological value. The specific gravity of the mixed secretion is on an aver- 

 age 1003, and its reaction is normally alkaline. The total amount of secretion 

 during twenty-four hours varies naturally with the individual and the condi- 

 tions of life; the estimates made vary from 300 to 1500 grams. Chemically, in 

 addition to the water, the saliva contains mucin, ptyalin, albumin, and inor- 

 ganic salts. The proportions of these constituents are given in the following 

 analysis (Hammerbacher) : 



In 1000 parts. 



Water 994.203 



Solids : 



{Mucin (and epithelial cells) 2.202 ^ 

 Ptyalin and albumin 1.390 V 5.797 

 Inorganic salts 2.205 ) 



Potassium sulphocyanide 0.041 



The inorganic salts, in addition to the sulphocyanide, which occurs only in 

 traces, consist of the chlorides of potassium and sodium, the sulphate of 

 potassium, and the phosphates of potassium, sodium, calcium, and magnesium ; 

 the earthy phosphates form about 9.6 per cent, of the total ash. Mudn is an 

 important constituent of saliva; it gives to the secretion its ropy, viscid cha- 

 racter, which is of so much value in the mechanical function it fulfils in 

 swallowing. This substance is formed in the salivary glands. Its formation 

 in the protoplasm of the cells may be followed microscopically (see the section 

 on Secretion). Chemically, it is now known to be a combination of a proteid 

 with a carbohydrate group (see section on The Chemistry of the llody). So 

 far as known, mucin has no function other than its mechanical use. The pres- 

 ence of potassium sulphocyanide (K( !NS) among the salts of saliva has always 

 been considered interesting, since, although it occurs normally in urine as well 

 as in saliva, it is not a salt found commonly in the secretions of the body, and 

 its occurrence in saliva seemed to indicate some special activity on the pari of 

 the salivary gland, the possible value of which has been a subjeel of specula- 



