SECTION I 

 DIGESTION IN THE MOUTH 



IT is a common experience that when food is taken into the mouth 

 there is a flow of a liquid, ' saliva/ into this cavity. Saliva 

 is the product of secretion of three pairs of large salivary glands 

 situated in the neighbourhood of the mouth and pouring their secre- 

 tions into this cavity by means of ducts 

 It is possible to collect the fluid secreted 

 by each of these glands separately, and it 

 is found that the saliva varies in pro- 

 perties according to the gland from which 

 it is derived. In addition to these large 

 glands the whole mucous membrane of 

 the mouth is beset with small glands. 



The saliva is in most cases a mixture 

 of the secretions of all three pairs of 

 salivary glands as well as of the small 

 glands of the mucous membrane. When 



salivary glands. " collected it forms a colourless cloudy 



a, sub-lingual gland ; &, sub- liquid, slimy in character. The cloudi- 

 ^Tl L opJ^Tbf ness is due to the presence of a number 

 ducts of sub-maxillary and sub- o f formed elements consisting of des- 



lingual glands; *, opening of . -. .., v -, -,. , . . 



duct of parotid gland. quamated epithelial cells, disintegrating 



leucocytes, and gland- cells, as well as 



coagulated clumps of mucin. Its reaction is in healthy individuals 

 slightly alkaline. Its specific gravity varies between 1002 to 1008. 

 Its chief constituents are coagulable proteins, mucin, and in some 

 cases a diastatic ferment, ptyalin, and traces of potassium sulpho- 

 cyanate. Its average composition is as follows : 



100 parts mixed saliva contain : 



lotal solids 0-5 to 1-0 



Inorganic solids . . . . . . . 0-4 to 0-6 



Organic solids (mucin, serum albumin, serum globulin) . 0-1 to 0-4 



Potassium sulphocyanate . . . . . . 0-00 to 0-016 



Freezing-point (A) = - 0-07 to - 0-34 



Potassium sulphocyanate is an almost constant constituent of human saliva, 

 though it is often absent in that of other animals, such as the dog. It is generally 



740 



FIG. 319. Dissection to display the 



