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 secretions 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 properties 



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 collected it forms 

 a colourless cloudy liquid, slimy in character. 

 The cloudiness is due to the presence of a 

 number of formed elements consisting of 

 desquamated epithelial cells, disintegrating 

 leucocytes, and gland cells, as well as coagu- 

 lated 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 sulphocyanate. Its average composition is as 

 follows : 



FIG. 333. Dissection to dis- 

 play the salivary glands. 



a, sublingual gland; 6, sub- 

 maxillary gland; c, parotid 

 gland; d, common opening of 

 ducts of submaxillary and sub- 

 lingual glands; i, opening of 

 duct of parotid gland. 



100 parts mixed saliva contain : 



Total solids 



Inorganic solids ....... 



Organic solids (mucin, serum albumin, serum globulin) 

 Potassium sulphocyanate ..... 



I'Yee/ing-point (A) = 0-07 to 0-34 



0-5 to 1-0 



04 0-6 

 0-1 0-4 

 0-00 0-016 



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 present 

 to the extent of *01 per cent., so that on the addition of a drop of ferric chloride to 



706 



