92 PRINCIPLES OF VETERINARY SCIENCE 



and kept under identical conditions also shows that digestion 

 is more complete in some than in others. 



(A) Digestion in the Mouth. — The first digestive change that 

 food undergoes occurs in the mouth. It is a chemical change 

 and is due to the secretion known as saliva. 



Saliva plays a very important role in the digestive process, 

 especially through its water, which lubricates the food for masti- 

 cation and swallowing. It is a fluid of alkaline or neutral reac- 

 tion and turbid and slimy appearance, which deposits a white 

 precipitate on standing. 



The amount of saliva secreted by a horse in twenty-four hours 

 has been estimated at 72 to 84 pounds, but will depend on the 

 dryness of the food consumed. This is nearly ten times as much 

 as the volume of urine voided during the same peiod. A feed 

 of 6 pounds of dry hay has been found to increase during 

 mastication to 24 pounds or to add four times its weight of saliva. 

 Colin places the daily secretion of saliva in the ox at 112 pounds. 

 Man secretes between 2 and 4 pounds daily. 



As all students in agricultural colleges have an opportunity to study Feeds 

 and Feeding it is to be presumed that they are familiar with the composition 

 and significance of the food of animals and their requirements under different 

 conditions. However, it is advisable to call attention to the general compo- 

 sition of foodstuffs, viz.: (I) the inorganic compounds — water and mineral 

 substances ; (II) the organic compounds — proteins and albuminoids, which are 

 body-building foods; and fats and carbohydrates, which are principally 

 fuel foods that supply work and heat energy. The food constituents and the 

 constituents of the animal body mentioned in Chapter I should also be 

 compared. 



The composition of saliva varies with the character of the sub- 

 stance 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 inorganic constituents are water to the 

 amount of 99 per cent., and phosphates, chlorids, and sulphates of 

 sodium, potassium, calcium, and magnesium. The organic 

 constituents are mucin and the enzyme ptyalin. The water 

 and salts are derived directly from the blood, while the mucin 

 and ptyalin are manufactured by the gland-cells. 



Mucin is found in very small amount in the saliva of the horse. 

 In that of man and certain other animals it is present in larger 

 amount. 



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