But a number of circumstances unite to prove that the gases of the stomach are in great part derived from some other source. Thus the quantity of air taken with the food can be but small. While percussion and aus cultation show, that the cavity of the healthy organ is often largely distended with gas. And the above analysis further points out, that not only is the increase of carbonic acid dispro portionate to the decrease of oxygen, and therefore (unlike the interchange in the skin and lungs) not due to a mere physical pro cess of diffusion, but that a new element, hydrogen, has been added to it.
The same arguments apply still more for cibly to those gases, which almost invariably distend the intestines. For during diges tion, they could hardly pass the pylorus; and at any other time would be very unlikely to enter the stomach', through which alone they could reach the duodenum. Hence in the case of the intestinal segments of the canal, we are referred almost exclusively to those sources which, we have already seen, will be necessary to explain the greater part of the gas present in the stornach.
2. Gases may be developed in the alimen tary canal from the decomposition of the food which it contains.
Difficult as it is to decide on the evidence at present before us, there seem to be valid reasons for regarding this as the process by which theintestmal gases are chiefly, if not ex clusively, set free in the alimentary cavity. The food introduced into this cavity is speedily converted into a decomposin,g mass, which is useful to the organism solely by virtue of the metarnorphoses it is undergoing. And though these metamorphoses generally seem to be limited to processes, by which elements are merely re-arranged in the solid or liquid form, and not given af as gases, still they' are easily susceptible of being carried further, so as to involve a more or less copious evolution of gaseous fluids.
Now, the putrefaction of the protein com pounds of the food, together with the fermen tation of its hydrates of carbon, would amply account for these gases ; as well as for the ammonia which has been alluded to as pro bably throwing down part of the soluble phosphates of the intestinal contents, in the form of crystals of the triple phosphate of am monia and magnesia. For not only are all the gases in the above analyses producible by the various processes of putrefaction external to the body, but their proportions to each other are precisely those which might be expected from the known composition of the food.
The conditions which favour the presence of these gases remarkably confirm this view. Too large a quantity of food—and especially of food that consists of substances which are either putrefying or fermenting, or are pecu liarly liable to undergo these changes —noto riously increases the amount of gases thus generated in the bowels. The liability of cattle to a dangerous distention of this kind, when surfeited with green food, is well known to agriculturists. And in like manner, an increased quantity of sulphuretted hydrogen is generally expelled in the flatus of animals which have been fed upon* beans, or made to take sulphur with their food. While the practice of naedicine acquaints us with the fact, that all circumstances which lower the tone of the alimentary canal, or lessen the energy of its secretions, further these sponta neous (though abnormal), metamorphoses of its contents ; and thus give rise to a corres ponding increase in the quantity of the gases which form their direct result. We may per haps find an additional confirmation of this view in a comparison of the various instances analyzed above. At least, the great devia tions which they exhibit, seem better expli cable by the variable composition of the food, than by any theory which would refer their development to the organism or the blood itself.
Finally, it is well known that the complete exclusion of food from the digestive cavity often gives rise to a peculiar white and con tracted state of the tube, which implies an entire absence of all such gaseous contents from the greater part of its length. This appearance is so generally seen in the bodies of animals after long fasting, as to constitute an important feature in the medico-legal evi dence of death by starvation.
3. It has been supposed that gases are set free in the intestinal canal by a kind of secre tion or transpiration frotn the blood.
But in alluding such a process, it is neces sary to premise that, strictly speaking, it would hardly deserve the name of a secretory act. Even assuming that it really discharged the gases of the blood into the intestinal canal, we should scarcely be warranted in terming their passage a true process of secretion. On the contrary, all analogy indicates that it would rather constitute an act of diffusion : — a diffusion which would probably obey the same laws, and exhibit somewhat of the same course, as that which chemistry has success fully investigated in the case of the lungs and skin.