CHAPTER VII. 



THE DIGESTIVE FLUIDS. 



As I have pointed out, the greater portion of the food-stuffs which 

 are ingested by animals cannot be utilized as such directly, but must 

 first be transformed into material that is capable of diffusing through 

 animal membranes. These changes occur in the gastro-intestinal 

 tract, and are effected by the secretions of the various digestive 

 glands, viz., the saliva, the gastric juice, the pancreatic juice, the 

 succus entericus, and the bile. 



THE SALIVA. 



General Characteristics. The saliva is the secretory product 

 of the salivary glands, viz., the parotid, the submaxillary, and the 

 sublingual glands, to which the secretion of the smaller mucous 

 glands of the oral cavity is further added. 



The saliva is a colorless, inodorous, tasteless, somewhat stringy 

 and frothy, opalescent fluid, which normally possesses a slightly alka- 

 line reaction and a specific gravity ranging between 1.002 and 1.008. 

 A slightly acid reaction may, however, also be observed, and is then 

 referable to the presence of lactic acid, which is formed through the 

 activity of micro-organisms, from food-material that has gathered 

 between the teeth or from desquamated epithelial cells. For this 

 reason also we find an acid reaction of the mouth-cavity on rising 

 in the morning. 



On microscopical examination the saliva is seen to contain a 

 variable number of pavement epithelial cells and so-called salivary 

 corpuscles. These are identical with the mucous corpuscles, which 

 are found in all mucous membranes, and represent young leucocytes 

 that have not entered the blood-current. They are derived from the 

 lymph-follicles of the mucous membrane, and in the case of the 

 saliva, no doubt, to a great extent from the tonsils. In addition 

 we find innumerable bacteria, and at times also schizomycetes and 

 moulds. On standing, the liquid becomes turbid, ow r ing to pre- 

 cipitation of calcium carbonate, which frequently also forms a fine, 

 iridescent film on the surface. This phenomenon is due to the 

 escape of carbon dioxide from the saliva, and explains the formation 

 of tartar on the teeth, as also the origin of the somewhat uncommon 

 salivary concretions in the larger ducts of the glands. 



Amount. The amount of saliva that is secreted in the twenty- 

 four hours varies somewhat even in health, but probably does not 



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