CH. XXX.] COMPOSITION OF SALIVA 479 



produced a similar effect ; in fact, the parotid secretion flows freely 

 if dry food is simply shown to the animal ; of course, in all such 

 experiments, the dog must be hungry. 



Such observations bring the salivary secretion into line with the / 

 other digestive juices ; they show the psychical element involved, and 

 point out also the wonderful adaptation of the secretory process to 

 the needs of the animal ; thus the submaxillary saliva, which is mainly 

 a lubricant in virtue of its mucin, flows whatever the food may be, 

 whereas moist food requiring no watery saliva from the parotid 

 excites the flow of none. 



Extirpation of the Salivary Glands. These may be removed 

 without any harmful effects in the lower animals. 



The Saliva. 



The saliva is the first digestive juice to come in contact with the 

 food. The secretions from the different salivary glands differ some- 

 what in composition, but they are mixed in the mouth ; the secretion 

 of the minute mucous glands of the mouth and a certain number of 

 epithelial cells and debris are added to it. The so-called "salivary 

 corpuscles " are derived from the glands themselves or from the tonsils. 



On microscopic examination of mixed saliva a few epithelial 

 scales from the mouth and salivary corpuscles from the salivary 

 glands are seen. The liquid is transparent, slightly opalescent, of 

 slimy consistency, and may contain lumps of nearly pure mucin. 

 On standing it becomes cloudy owing to the precipitation of calcium 

 carbonate, the carbonic acid, which held it in solution as bicarbonate, 

 escaping. 



Of the three forms of saliva which contribute to the mixture 

 found in the mouth the sublingual is richest in solids (275 per cent.). 

 The submaxillary saliva comes next (21 to 2'5 per cent.). When 

 artificially obtained by stimulation of nerves in the dog the saliva 

 obtained by stimulation of the sympathetic is richer in solids than 

 that obtained by stimulation of the chorda tympani. The parotid 

 saliva is poorest i'n total solids (0'3 to 0*5 per cent.), and contains no 

 mucin. Mixed saliva contains in man an average of about 0'5 per 

 cent, of solids : it is alkaline in reaction, due to the salts in it ; and 

 has a specific gravity of 1002 to 1006. 



The solid constituents dissolved in saliva may be classified thus : 



( a. Mucin : this may be precipitated by acetic acid. 



n ) b. Ptyalin : an amylolytic ferment. 



'I c. Proteid : of the nature of a globulin. 



V. d. Potassium sulphocyanide. 



( e. Sodium chloride : the most abundant salt. 



Inorganic . /. Other salts : sodium carbonate, calcium phosphate and 



\ carbonate : magnesium phosphate ; potassium chloride. 



