When water is drunk by a horse in the usual way, it passes through the gullet, stomach, and small intestine, which is able to absorb fluids, and the remainder flows into the caecum. This transit from mouth to caecum is rapidly effected, and consequently with little loss of water. The stomach of the horse, according to Colin, has hardly any absorbent power. The caecum, the walls of which can freely absorb water, is constructed in such a manner that it can hold a considerable quantity of fluid; the excess being passed out along with the food, into the lower portion of the large intestine. As the absorbents take up water only to the extent required by the body, the caecum acts as a water reservoir to the system. It appears that the fact of the horse possessing this water reservoir prompts him, when he is thirsty and when he has the opportunity, to take a copious drink, so as not only to supply his present need, but also to lay up a store for the requirements of the next few hours. We also find that, unlike the majority of human beings, he does not as a rule drink frequently during the twenty-four hours, supposing that he has free access to water. We may therefore conclude that it is in harmony with a horse's nature for him, under ordinary conditions, to drink copiously when he is thirsty, so as to fill his caecum. To restrict him to small and frequent drinks, would be as irrational as it would be to apply the same method to the feeding of dogs, whose health is best maintained when their comparatively large stomach receives food only once or twice a day. When the water in a horse's system suffers from a rapid and continuous drain, as for instance, during long and severe work, the intervals between watering ought to be proportionately decreased.

Absorbed water is excreted chiefly by the skin, kidneys, lungs, and in the recently delivered mare, by the udder. Un-absorbed water is passed out with the dung.