The above view, as to the share which both the biliary and the intestinal constituents take in producing the colour and odour of the fmces, appears so irrefragable, that we may content ourselves with a passing allusion to those experiments by which it has been at tempted to establish the predominant or exclu sive influence of either. Thus, while it has been pointed out by Valentin + that putre fying bile diffuses the strongest smell of or dure, Liebigt states that he has succeeded in the artificial production of the fmcal odour by a process which essentially consists in imper fectly oxidizing sonie of the more azotized tis sues of the body. The latter experiment has been regarded as leading to the inference, that certain effete constituents of the blood are secreted into the intestine, in a like state of partial oxidation. But, even could we assume the chemical identity of two substances merely from their having the same overpowering smell, we should still be left in uncertainty, as to whether these odorous matters were excreted directly from the blood into the bowel, or were introduced into it indirectly, by means of the secretion and subsequent metamorphosis of the bile. The very large intestinal constituent of the meconium, associated as it is with an almost inodorous character of this excrement, would indicate that, on the whole, Valentin's view of the biliary origin of the fiecal odour is the more correct one. At present, however, a satisfactory decision of the question seems impossible.
But whether the peculiar odour of the fkces be biliary or intestinal, there can be no doubt that it is derived, in the first instance, from the blood. For the smell of the excrement of any particular species always has a close relation to that odour, which is specific to the body of the animal, and which appears, in va rious degrees of intensity, in all its different excretions. And it is even stated by Wehsarg* to present differences specific to the individual.
Finally, we need have little scruple in as serting, that all the physical properties of the fmces are also in a great measure dependent on that alimentary residuum which usually enters so largely into their composition. The quan tity of fatty matter and of casein usnally pre sent in the excrement of the sucking-child, the deepening (and finally black) colour of the fxces in persons who feed chiefly on vege tables, the lactic acid found in the evacuations of carnivora, or the oil which may often be de tected in the stools of persons by whom even small doses of cod-liver oil are being taken medicinally — form instances of this kind, which might obviously be multiplied to ahnost any extent. Nor is the process always li mited to a mere admixture or decomposition of the food itself. On the contrary, the metamorphoses which most of its ingredients have to undergo, often react on the secretory contents of the canal, so as to modify their appearances by the addition of properties more or less foreign to them. And nothing but that comparatively uniform admixture of the chief alimentary principles of the food, which we shall hereafter find is absolutely necessary to the life of the individual, will account for even the imperfect uniformity traceable in examining the excrements of large numbers of individuals.
The reaction of the human fmces is gene rally acid sometimes neutral or alkaline. The quantity 'daily evacuated by a healthy male adult may be estimated as amounting, on an average to about five ounces avoirdupois.
The specific gravity of the fces is gene rally greater than that of water, °win,* to the solids which they contain. But it is'far too variable to allow of any average estimate being made. For it varies, not merely with the bulk and weight of the alimentary resi due that forms so large a portion of the or dinary excrement, but also with the degree in which the faecal mass has been condensed by the absorption of its watery constituents. And it would further seem, that the fieces are capable of being partially dried, and rendered much lighter, by a mechanical admixture of intestinal gases with their substance while still within the body. At least it is very common for different portions of the same evacuation to exhibit very different specific gravities : — the first portions of the excre ment, which previously occupied the lower extremity of the rectum, being much lighter than water; while those subsequently extruded, though less solid, are so much heavier, as to sink rapidly in this liquid.
The mechanical composition of the excre ment might almost be deduced from what has already been said of its origin. A large quantity of its mass no doubt consists of un digested food.* This must, however, be sub divided into two parts, which have a very different import with respect to the digestive function. One of these, which is usually much the larger, includes all those substances that are incapable of being dissolved by the various secretions poured into the intestinal canal. Such are the harder parts of various animal and vegetable tissues : — the sarco lemma of muscular substance, the cells of car tilage, fragments of bone, the elastic fibres of areolar tissue ; together with the husks, shells, pods, chlorophyll, epidermis, and various dense membranes, cells, vessels, and fibres of the various fruits and seeds used as food. Some of these tissues quite defend the soluble contents they enclose. The other portion consists of substances which, though really capable of solution in the alimentary canal, have escaped this process :— whether from having been taken in too large a quantity, from not having sojourned in the tube during a sufficient interval of time, or from having been exposed to secretions which are par tially devoid of their proper solvent force. Of these three causes of such an admix ture, the first is the more common, and pro bably constitutes an invaluable safeguard against the dangerous results which might otherwise follow every act of over-eating. Hence in the state of repletion, whether relative or absolute, large quantities of fat, muscular fibre, albumen, casein, starch cells, fibrous tissue, and other strictly alimentary substances, escape digestion, and are found in the fmces. And conversely, it is highly probable that individuals (as well as animals) may have their digestive powers so raised by hunger and need, as to be enabled to extract nutriment from substances tIat would other wise clef/ the action of their gastric and in testinal juices. In short, these undissolved and insoluble—undigested and indigestible— constituents of the alimentary residuum niay almost be said to merge into each other, ac cording to the habits of the individual with respect to the ingestion of food.