The elementary analyses of the fzeces hither to made possess little physiological signifi cance, or general validity. But from what has already been stated, it is obvious that the entire excretory part of the ordure removes from the body very little water or nitro gen ;—probably not more than ,;,,th or ,t6th of that quantity of each of these elements which is daily excreted in the urine.
The time during which the contents of the intestinal tube sojourn in its different segments is probably a very uncertain as well as variable one. In diarrhcea, the whole canal is some times traversed by these contents in two hours ; while in obstruction, weeks or months may elapse without their complete transit. The mean rate which lies between these two morbid states can only be conjectured. But there are reasons for supposing, that the food of a healthy adult occupies about twelve hours in passing through the small intestine. While from thirty-six to sixty hours may be assumed as its average sojourn in the large intestine, prior to its ultimate expulsion from the rectum.
Intestinal gases.— In speaking of the elastic fluids which are generally contained in the large intestine, and are occasionally expelled from its lower orifice, it will be advantageous to contrast then: with the gases found in other parts of the alimentary canal : — viz., in the stomach and the small intestine. Many years ago the composition of these gaseous contents of the canal was correctly given by Jurine, from an examination of the corpse of an idiot soon after death by cold. But it is to Magendie* and Chevreul that we owe the only trustworthy quantitative analyses on the subject. Their observations were made upon the gases found in the bodies of criminals im mediately after their execution. Some authors have therefore thought it worth while to al lude to their results, as being probably affected by the dyspepsia which the dread of such an impending doom might be supposed to have produced in these unhappy persons. With out, however, assigning any definite value to this contingency, it is enough to say that they still remain far preferable to any other such analyses :—to those, for instance, of Chevillott, whose rather different results are quite ex plained by the time after death to which his examinations were deferred, and the decom position which had therefore begun, both in the tissues of these corpses, and in the ali mentary and secretory contents of their in testines.
We may best arrange these analyses in the following tabulated form : — It is only from such analyses that we can form any reasonable inference as to the origin of the gases to which they refer.
In making such an inquiry, four sources of aeriform matter at once suggest themselves ; either of which seems at first sight capable of at least partially explaining the presence of gaseous substances in the digestive canal. And the claims of each of these must be separately examined before we can conjecture the proba ble amount of its product, or its share in those reactions which the physical properties of gaseous fluids so easily allow them to excite.
1. Air may be introduced into the intestinal canal from without the body. Just as some of the lower animals can distend the abdomen by a voluntary deglutition of air, while even the higher Mammalia have been noticed to fill the stomach with air by the movements which precede the act of vomiting, so per sons have been observed to swallow air, and afterwards expel it by eructation. And apart from such exceptional cases, there is good reason for believing that the ingestion of food always introduces into the stomach an ap preciable quantity of atmospheric air ; part of which is perhaps mechanically carried down with the alimentary bolus, while another part enters the organ in a state of more minute di vision, with the frothy saliva.
The air which is thus introduced into the stomach will doubtless here undergo a certain amount of difflision or interchange with the elastic fluids dissolved in the liquid blood that circulates in the capillaries of the organ. And this diffusion probably imitates that which takes place between the air and the blood at the surface of the lungs and skin. It will therefore convert the gaseous mixture of the atmosphere into one containing less oxygen, and more carbonic acid ; the extent of the change in both these respects varying chiefly with the duration of its sojourn in the stomach.