Quotes G.A. Müller and A. Glass, Diseases of the Dog and their Treatment (3rd Ed.) (Chicago: Alexander Eger, 1911).
Description:

'It must be borne in mind that the digestive juice in a dog's "interior" is largely composed of hydrochloric acid - that being essential to the dissolving (as it were) of the food. It follows that there may be, and often is, not too much acid but too little acid in the gastric juice, and that what a dog may be suffering from is not acidity, but alkalinity - the very opposite. This subject will be found dealt with at length in the big work by Müller and Glass on "Diseases of the Dog," and I venture to quote from that some facts showing how different is the action of the dog's digestive system on different sorts of food. The facts are based [on the] wisdom of not feeding the average dog more than twice a day:-

After taking an average meal of meat cut in small pieces the digestion in the stomach is very active and free; it increases until the third hour and slowly decreases until the ninth, and is nearly over at the twelfth hour. After eating a very large meal the digestion is somewhat slower and lasts considerably longer, the different kinds of meat also vary in the time of their digestion. Pork is the easiest to digest and others are classified in the following order: mutton, veal, beef, and lastly the flesh of other animals; skin, tendon, sinew, cartilage, and bones arevery hard to digest; the latter are digested from their surface and are reduced as the gelatinous parts are acted upon and dissolved and the lime salts remain unchanged. Fat meat is harder to digest than lean; fat undergoes no change in the stomach, but passes on and is digested in the intestines. The gastric juice acts on and reduces roasted meats and if raw meat is chopped up in small pieces the gastric juice acts on it much more quickly. It has never been satisfactorily settled whether raw or cooked meat is easier to digest.

Upon the latter point I would observe that meat which is thoroughly cooked is more likely to be deprived of its vitamin content - that is why for my own part I always advocate either uncooked or lightly cooked meat.

...

Milk, we are told, is comparatively slow in digestion. Four hours after an animal had taken a quantity of milk he was still found to have a considerable quantity of cheese in his stomach. But hydrocarbons (starchy foods) provide further illustrations. The same authorities say:-

Five hours after a meal consisting of rice and potatoes the mass was liquefied and softened; the mashed portion of the potatoes had disappeared, but the lumps remained. After a meal of rice, the following observations were made: After one hour 10 per cent, was digested; [...] after two [four] hours 82 per cent.; after six hours 90 per cent.; after eight hours 99 per cent., and at the end of ten hours it had entirely disappeared.

The authors cite Professor Hoffmeister as their authority for these figures; and they go on to say that both he and Professor Ellenberger (another famous Continental specialist)-

have come to the conclusion that rice is chiefly digested in the intestines, as there is so much muriatic (hydrochloric) acid in the stomach immediately after eating that saccharation cannot take place; and also that the dog swallows his food with so little mastication that the saliva has no time to make any change in the starch.

This is important in considering how much biscuit or other cereal food a dog should be allowed, and at what intervals of time. We learn that-

the digestion of starch is impaired by an oversecretion of hydrochloric acid (hyperacidity); this condition, according to the researches of Ellenberger and Hofmeister, is not of great importance, although in many casess it is frequently seen in  ulceration and in acute and chronic catarrh of the stomach, in "nervous dyspepsia," so common in man, but not seeming to occur in the dog, sometimes in carcinoma and other serious stomach diseases. [orig: 'this condition, according to the researches of Ellenberger and Hofmeister, is not of great importance, although in man it is frequently seen in ulceration and in acute and chronic catarrh of the stomach. "Nervous dyspepsia," so common in man, does not seemto occur in the dog.']

Carcinoma is a malignant cancerous condition which very often affects the mammary glands of old bitches - that is by the way.'