The expulsion of the feces take place when they arrive in the rectum, which is speedily irritated by their presence ; and is performed partly by the muscular coat of that intestine, and partly by the mus cles of respiration, producing the effort called straining. The periods of voiding the excrement vary, from several causes : they are more frequent in the young sub jects, where the stools are more liquid. In the adult they should not be less, in a healthy state, than once in twenty-four hours.
Urinary Secretion. The liquids which we drink, absorbed by the lacteals, toge ther with the nutritive part of the solid aliment, dilute the latter, and serve as a vehicle for it. They increase the quan tity of the blood, and render it more fluid ; conveyed into every part of the circulating system, they penetrate all our organs, carry away the particles detached from them in the different vital processes, and are then separated from the mass of fluids by the urinary organs, together with various other substances, whose re tention in the body would produce inju rious effects. The kidneys, therefore, dispose of the residual part of our liquid ingestu, as the feces are formed by the more solid foods, and the quantity of urine may, of course, be expected to vary according to that of the drink. All the old parts of the frame, which are con stantly removed by the absorbents, while new depositions are formed by the nu trient arteries, go off in the same way ; and hence the urine, although apparent ly a watery fluid, and called in common language water, contains a great deal of animal matter.
From the above account it will be rea dily understood, that the properties of the urine must vary according to the time at which it is voided after meals, the quan tity of food and drink, the age and com plaints of individuals, &c. Physiologists have distinguished urine of the drink, chyle, and blood. The first is a watery fluid, almost colourless, evacuated very soon after drinking, and possessing very slight urinous characters; the second, evacuated two or three hours after meals, is better elaborated, but not yet complete in its constituent principles ; the last, voided after the repose of the night, has all the properties of urine in an eminent degree. In infants it possesses no phos phate of lime nor phosphoric acid, as those substances are employed in the bu siness of ossification, which is then active.
In old persons, on the contrary, where the bony system, already overcharged with phosphate of lime, refuses to admit any more, this substance is carried off by the kidneys. It is removed in the same way in rachitis and mollities ossium, where the bones become softened by its ab sence.
The great quantity of saline and crys tallizable elements contained in the urine, account for the frequency of calculous concretions, which are found by recent and accurate analysis to vary very consi derably in their composition. As there it no substance in the body which may not be evacuated by the urine, and mani fest itself in that liquid, so, unait diffe rent circumstances, every thing possess., ing a power of concretion may beeome the subject of urinary calculus. This di versity of constituent elements, together with the want of characteric symptoms of the different species, and the irritation which the coats of the bladder must ell, perience from chemical reagents, will' convince us how extremely difficult, if not impossible, it must be, to discover a lithontriptic that would obviate the ne cessity of a surgical operation.
The urine is very speedily and sensi bly affected by certain substances ; thus asparagus occasions a remarkable fcetor in this fluid : and turpentine imparts to it a violet odour. For a further account of its composition and physiology, see the article therrix.
Almost every physiologist has noticed the rapidity with which this secretion is carried on : aqueous fluids, taken by the mouth, are sometimes separated so quick ly by the kidneys, that an immediate communication has been suspected be tween the stomach and kidneys, on the supposition that there had not been a suffi cient time for the fluid to arrive at the latter organ in the regular course of ab sorption and circulation. This conjecture derives no countenance from anatomy, and the size of the renal vessels ex plains the fact without any such suppo sition.
Absorption, or the process by which the chyle, separated from the food by the digestive organs, is carried into the blood, naturally follows the account of diges tion. We have very little to add to what is stated on this subject in the article