We have seen that for every 2 1/2 lb. of hay which a horse eats, his salivary glands have to pour into his mouth a gallon of saliva (10 lb.); and for the same weight of oats, more than a quart of saliva. Hence, 6 gallons of saliva would be required for the mastication of a daily ration of 12 lb. of hay and 11 lb. of oats, without counting the saliva which is continually secreted while the animal is not feeding. There are also large quantities of gastric juice, pancreatic juice, bile, and intestinal juice secreted. Considering the composition of these digestive fluids, we may safely say that the water contained in their daily supply to the alimentary canal, is on an average a good deal more than double the quantity of water drunk by the horse in the twenty-four hours. It is therefore evident, especially when we note the large expenditure of water from the skin, lungs, and kidneys, that a considerable portion of the water contained in the outpoured digestive fluids becomes reabsorbed before it leaves the system. I need hardly explain that all the water in the digestive juices is derived from the blood. This question of the reabsorption of water does not affect the fact that the animal needs his blood to be fully supplied with water in order that it may secrete a sufficiency of digestive juices. Consequently we should give a horse as much water as he chooses to drink before feeding him.

Absorption

As the absorbents are separated from the contents of the intestines by only a thin membrane, they readily take up water, digested food and dissolved salts, and convey them into the blood after having passed them through the liver and other glands, which produce changes in the food that are necessary for rendering it capable of being utilised by the body. The absorption of digested food begins in the small intestine and is completed in the large one, a certain amount of water and dissolved salts being absorbed in the stomach. The absorbed food may be regarded as that which is taken up by the tissues (assimilated); and that which, being apparently in excess of the requirements of the system, is passed out of it without undergoing change. Thus, egg albumen is often present in the urine of human beings who have recently eaten large quantities of such albumen. Also, common salt is found in the perspiration of horses and men; and when water has been drunk in excess, an increased discharge of urine generally takes place.