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THE HORSE IN HEALTH AND DISEASE 



(A) Digestion in the Mouth. — The first secretion with which 

 the food comes in contact is saliva. This is a fluid of alkaline or 

 neutral reaction and turbid and slimy appearance, which deposits 

 a white precipitate on standing. Besides the 99 per cent, of water 

 which it contains, there are present leukocytes, epithelial cells, a 

 large number of bacteria, and phosphates, chlorids, and sulphates 

 of sodium, potassium, calcium, and magnesium. The composition 



Fig. 22. — The principle of glandular structure. In the upper figure a 

 simple microscopic gland has been laid open by cutting through its vertical 

 axis. The cells are seen to surround a recess, or alveolus, into which they dis- 

 charge their secretion. Below, the same structure is shown in its entirety, and, 

 in addition, the encircling blood-vessels, which contribute to make good the loss 

 due to the activity of the secreting cells. (Stiles, Nutritional Physiology.) 



of saliva varies with the character of the substance in the mouth 

 which has excited it to flow. Coarse sand or irritating chemicals 

 cause a flow of very watery saliva to wash the irritant away; 

 water excites the flow of a very viscid saliva rich in mucin. 



The main uses of saliva are to lubricate the food for mastica- 

 tion and swallowing and to aid in digestion. Ptyalin, the salivary 

 enzyme, is the principal constituent of saHva from the standpoint 



