The process of digestion is performed much more fully in the intestines than in the stomach. When food leaves the stomach and enters the small intestine, it becomes mixed with bile and pancreatic juice, which flow into the intestine by a common opening. The pancreatic juice of the horse contains, according to Leuret and Lassaigne, about 99.1 per cent. of water. ' Pancreatic juice possesses ferments which respectively dissolve nitrogenous matter and starch, and which split up fat in such a manner that the fat can be readily absorbed. Bunge states that "the pancreas is the digestive gland par excellence."

Halliburton considers that the chief use of bile is to aid the pancreatic juice in the digestion of fat. The results of many experiments (see Colin's Physiologie Comparee) suggest the conclusion that bile is much more concerned in the absorption of fat than is pancreatic juice. It is doubtful if bile has any antiseptic power; for it cannot prevent even itself from decomposing. When the secretion of bile is deficient in quantity, the intestines during digestion contain a large amount of unabsorbed fat which encloses a portion of the nitrogenous matter of the food and thus prevents it from becoming absorbed; the result being that the nitrogenous matter becomes putrid, and consequently imparts a foul smell to the dung (Bunge). Colin estimates that a horse secretes daily about one-sixty-sixth of his weight of bile. This secretion contains from 82 to 91 per cent. of water; various salts, chiefly of soda; fatty matters; and colouring matters.

The liver constantly secretes bile, which in men, cattle, and in the large majority of other mammals, flows into a reservoir (gall bladder) and from thence issues into the small intestine. As horses were found to have no gall bladder, and as they were known to feed more or less continuously when at grass, it was generally inferred that the absence of a gall bladder in a horse was a proof that he should be fed at frequent intervals of time. Although the conclusion was true, the premises of the argument were incorrect; because the possession or non-possession of a gall bladder appears to have no connection with the digestion of food. For instance, among ruminants we find that deer, camels, and llamas have no gall bladder. This reservoir is also absent in ostriches, parrots, guinea fowl, and pigeons (Colin), though present in most other species of birds. Besides, the possession of a gall bladder does not prevent the continued flow of bile into the small intestine, whether the food is or is not passing through that portion of the alimentary canal.

On leaving the small intestine, the food and water become collected in the caecum, which acts to some extent as a supplementary stomach. In it and in the greater portion of the remainder of the large intestine, the processes of digestion and absorption are continued, until finally the unappropriated residue in the form of dung is expelled. The digestive power of the lower portion of the large intestine is feeble. The intestinal juice has the property of dissolving carbohydrates and nitrogenous matter. In the horse, a portion of the woody fibre is broken up in the large intestine by the action of bacteria which are brought into the body along with the food; the result being that the woody fibre which undergoes this change becomes converted into carbonic acid, marsh gas, acetic acid, and butyric acid. Bunge explains that this carbonic acid is absorbed by the intestines and is breathed out by the lungs; but that the marsh gas remains in the intestines until it is passed out in the form of flatus.

As the products of this reaction cannot be utilised by the system, this breaking up of woody fibre by bacteria must not be regarded as digestion.