ESSENTIALS OF PHYSIOLOGY. 



lateral, and antero-posterior movements of the jaws, the various frag- 

 ments being directed in turn by the muscular movements of the cheeks, 

 lips, and tongue, so that they come between the opposing teeth. At 

 the same time, saliva is being poured out in considerable quantity, and 

 is being intimately mixed with the food by the same grinding process. 

 After mastication the insalivated food is collected into a bolus on the 

 tongue by further movements of the cheeks, lips, and tongue itself, and 

 in this condition it is ready for the process of swallowing. 



The Composition of Saliva. Saliva, which is the mixed secretion of 

 the parotid, submaxillary, and sublingual glands, together with that of 

 the buccal glands scattered about the mucous membrane of the mouth, 

 is a viscid, colourless, cloudy fluid, with a slightly alkaline reaction, and 

 an average specific gravity of about 1005. It contains as a rule over 

 99 per cent, of water and less than 1 per cent, of solid constituents. 

 The latter consist of coagulable proteins, mucin, a diastatic enzyme 

 called ptyalin, and inorganic salts. Calcium salts are present in con- 

 siderable proportion, and are responsible for the formation of tartar on 

 the teeth. Traces of potassium thiocyanate are often present, and may 

 be recognised by the red colour which they give with ferric chloride. 

 The viscidity of saliva is due to the mucin it contains ; the cloudiness 

 is the result of the presence of numerous squamous epithelial cells from 

 the mucous membrane of the mouth, and of the so-called salivary cor- 

 puscles. The latter are granular spherical cells resembling leucocytes, 

 some of which are probably derived from the lymphoid tissue of the 

 tonsils, while others may come from the salivary glands themselves. 



The Functions of Saliva. The saliva serves other purposes besides 

 that of digestion, and in the dog it serves other purposes only, there 

 being no ptyalin in the saliva of that animal. By its admixture with 

 the food it facilitates mastication and deglutition. By keeping the lips 

 and t6ngue moist, it is of service in those movements which are essential 

 to the function of speech ; and, by dissolving substances which affect 

 the sense of taste, it renders these capable of stimulating the gustatory 

 end-organs. 



Tn these cases the use of saliva is a mechanical one, whereas in its 

 digestive function it gives rise to chemical change. Ptyalin acts upon 

 starch, converting it into dextrin and maltose. Raw starch is not 

 acted upon by saliva, which cannot dissolve the cellulose capsules of the 

 starch granules, and to enable ptyalin to produce its effect upon starchy 

 foods, these must be boiled or otherwise cooked. When starch granules 

 are heated in the presence of water, they swell, and the cellulose 

 capsules are ruptured so that the starch passes into a pseudo-solution. 



If a little saliva is mixed with some boiled starch paste : in a test- 



