CHAPTER III. 



SALIVARY DIGESTION. 



THE saliva is secreted by three pairs of glands, the submaxillary, 

 sublingual and parotid, reinforced by numerous small glands called 

 buccal glands. The saliva secreted by each pair of glands possesses 

 certain definite characteristics peculiar to itself. For instance, in 

 man, the parotid glands ordinarily secrete a thin, watery fluid, the 

 submaxillary glands secrete a somewhat thicker fluid containing 

 mucin, while the product of the sublingual glands has a more muci- 

 laginous character. The saliva as collected from the mouth is the 

 combined product of all the glands mentioned. 



The saliva may be induced to flow by many forms of stimuli, such 

 as chemical, mechanical, electrical, thermal and psychical, the nature 

 and amount of the secretion depending, to a limited degree, upon 

 the particular class of stimuli employed as well as upon the character 

 of the individual stimulus. For example, in experiments upon dogs 

 it has been found that the mechanical stimulus afforded by dropping 

 several pebbles into the animal's mouth caused the flow of but one 

 or two drops of saliva, whereas the mechanical stimulus afforded 

 by sand thrown into the mouth induced a copious flow of a thin 

 watery fluid. Again, when ice-water or snow was placed in the 

 animal's mouth no saliva was seen, while an acid or anything pos- 

 sessing a bitter taste, which the dog wished to reject, caused a free 

 flow of the thin saliva. On the other hand, when articles of food 

 were placed in the dog's mouth the animal secreted a thicker saliva 

 having a higher mucin content a fluid which would lubricate the 

 food and assist in the passage of the bolus through the oesophagus. 

 It was further found that by simply drawing the attention of the 

 animal to any of the substances named above, results were obtained 

 similar to those secured when the substances .were actually placed 

 in the animal's mouth. For example, when a pretense was made 

 of throwing sand into the dog's mouth, a watery saliva was secreted, 

 whereas food under the same conditions excited a thicker and more 

 slimy secretion. The exhibition of dry food, in which the dog had 

 no particular interest (dry bread) caused the secretion of a large 



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