PROBLEMS OF DIGESTION. 545 



the quantity of insoluble starch is greater than that of soluble starch, 

 gum, dextrin, or sugar ; in cooked meat, poultry, fish, and eggs, and 

 also in cheese, the albuminoid constituents are all solidified ; the vege- 

 table gluten and legumin are either solid, or are coagulated by cook- 

 ing; and even the fluid or finely granular casein of milk, is first pre- 

 cipitated or curdled in the stomach, by the action of the acid gastric 

 juice. Indeed, undissolved, though minute, granules of amyloid, in- 

 soluble oleoid, and solidified albuminoid substances, constitute the 

 most nutritive forms of food. 



In a chemical sense, these substances are instable compounds ; they 

 have a high atomic constitution, and are easily broken up by powerful 

 chemical agents, by elevated temperatures, fermentation, or putrefac- 

 tion. Nevertheless, under ordinary circumstances, they are insoluble 

 in water at the heat of the body, and are decomposable, or rendered 

 soluble, only by the action of agents and temperatures, which would 

 be destructive to living animal tissues. Thus, starch is rendered mu- 

 cilaginous only at the temperature of 160 ; it is changed into dextrin 

 at a still more elevated temperature ; and it is convertible into a sugar, 

 by the highly corrosive sulphuric acid. Of the fats, margarin and 

 stearin become fluid only at temperatures higher than that of the body, 

 viz., 114 and 118 ; none of them are easily miscible with, or can be 

 kept suspended, in minute particles, in watery fluids ; and to render 

 any of these soluble in water, they must be saponified by the action of 

 caustic alkalies, which are destructive to living tissues. The solid 

 albuminoid principles, so far from being soluble even in boiling water, 

 have their component particles knit still more firmly together, by being 

 boiled ; and putrefaction alone will dissolve them, a condition incon- 

 sistent with their retention of nutritive properties, and, indeed, con- 

 verting them into noxious products. 



The first problem of digestion, however, is to render such substances, 

 which, in this point of view, are refractory, soluble at a temperature, 

 and by means of agents, compatible with the life and integrity of the 

 digestive organs themselves. But, secondly, starch, even when dis- 

 solved, so as to form a soluble mucilage, and also albumen, when per- 

 fectly soluble, as in the white of egg, are too tenacious to pass readily 

 through moist membranes, and belong to the so-called colloid bodies, 

 which have a feeble permeating power, in comparison with the so-called 

 crystalloid substances (Graham) : whilst oil, likewise, passes through 

 moist membranes only under considerable pressure. Accordingly, in 

 the process of digestion, starch is not only dissolved, but is converted 

 into the crystalloid, and highly permeating substance, sugar ; albu- 

 minoid bodies are converted into a substance named albuminose, which, 

 though not shown to be crystallizable, nevertheless, permeates moist 

 membranes with great facility ; whilst fatty matters are either emul- 

 sified, decomposed, or dissolved. These transmutations are daily ac- 

 complished, within the body, at its proper temperature, in modes at 

 present only hypothetically explained, by the respective actions of the 

 salivin, pepsin, pancreatin, and conjugated fatty acids, of the saliva r 

 gastric juice, pancreas, and bile, 



35 



