DIGESTION 163 



It is well known in the human subject that fear or anxiety 

 gives rise to impulses inhibiting secretion of saliva ; the mouth 

 becomes dry, and the tongue refuses to move. In the horse an 

 identical condition is produced by abdominal pain. During an 

 attack of colic the mouth is quite dry, which symptom is of the 

 utmost value. The dryness is not due to thirst, for the animal 

 never drinks while the pain lasts ; a moist condition of mouth and 

 a desire for water are two favourable indications of the utmost 

 value in prognosis. 



Physical and Chemical Characters. — Mixed saliva is an alka- 

 line, opalescent, or slightly turbid fluid which readily froths 

 when shaken. On standing exposed to the air, it throws down 

 a deposit of carbonate of lime due to the loss of its carbonic acid. 

 It has a specific gravity of 1005 in the horse, and 1010 in the ox. 

 Examined microscopically, saliva is seen to contain epithelial 

 scales and salivary corpuscles. The latter are small round 

 granular cells which seem to be altered leucocytes, and are prob- 

 ably derived from the soft palate. About 06 per cent, of the 

 saliva consists of mineral matter, and 02 per cent., more or less, 

 of organic matter, the latter consisting of mucin (which gives 

 saliva its well-known viscidity and ropiness), and small amounts 

 of proteid substances the nature of which has not been exactly 

 determined. Mucin belongs to a peculiar group of proteid bodies 

 combined with a carbohydrate, for which see Chapter XX. 

 Ptyalin or salivary diastase is the most interesting organic con- 

 stituent of saliva in man, and belongs to a group of ferments 

 known as unorganised. It is doubtful if it exists in the herbivora, 

 and under any circumstances its amount has not been deter- 

 mined. Ptyalin is also absent from the saliva of the dog. The 

 salts of saliva are principally carbonate of lime, alkaline 

 chlorides, and phosphates of lime and magnesia. A substance 

 known as sulphocyanide of potassium has been found in minute 

 quantities in the saliva of the human subject, but is absent from 

 that of the horse. The gases of the saliva are principally carbonic 

 acid, with traces of oxygen and nitrogen ; there is no body fluid 

 which contains so much carbonic acid as saliva (65 volumes per 

 cent.). The three salivas have different physical properties : 

 Parotid saliva is watery, clear, and free from mucin, but contains 

 a small quantity of protei^ ; submaxillary and sublingual saliva 

 are viscid, especially the latter, owing to their richness in mucin. 

 The watery saliva is to impregnate the food and prepare it for 

 digestion, but the viscid salivas are principally concerned in 

 swallowing. 



Amount of Secretion. — Colin observed some remarkable facts 

 regarding the secretion of the various salivary glands in the 

 herbivora. When an animal is masticating, the parotid on that 



