It would appear, therefore, that the noxious property belongs to certain combinations only of these elements, which are formed in the course of the chemical changes in living beings, and which, when once formed, must either be ex pelled from the body, or else laid up in cells appropriated for the purpose, (as in the case of the resins and volatile oils in vegetables, and of the bile in the gall-bladder in animals,)and kept out of the mass of the nourishing fluid.
There is one general fact, on which much stress has been justly laid by Dr. Prout, which is confirmed by M. Raspail, and which may, perhaps, be concerned in determining the noxious qualities of certain compounds, in liv ing beings, viz. that although the elements which enter into the composition of organized bodies, readily combine, in other circumstances, so as to form crystals, yet the peculiar combi nations which they form in all the textures which are essential constituents of those organic structures are never crystalline. When a crystal occurs in an organized body, according to Dr. Prout,t it is always either the result of disease, or of some artificial process, or it is part of an excretion, separated from the nourishing fluid and from the useful textures.! Every one of these textures contains, even in its minutest particles, saline and earthy, as well as animal or vegetable matter; § but the combinations are always so arranged, by the powers of life, that these saline and earthy particles are always dif fused through membranes, fibres, or cells, never concentrated in crystals. On the other hand, the elements constituting the peculiar matters of the excretions are generally in such a state of combination as readily to assume the crystalline form, either alone, or in the simplest farther combinations of which they are susceptible ; and it seems possible, that this circumstance may be part at least of the cause which necessi tates their expulsion. This is only matter of speculation, but that some such general prin ciple determines the incompatibility of the mat ters of the excretions with the life of the struc tures in which they are formed, can hardly be doubted.
II. Although the necessity of various excre tions is obvious, there is a difficulty, both in the case of animals and vegetables, in fixing on those products of vital action which come exclu sively under this denomination ; and it appears certain, that some of the organs of excretion (such as the lungs) are at the same time de-. stined to other purposes, particularly absorption ; and even that part of certain excreted fluids (such as the bile) is employed likewise in the work of assimilation. But it is certain that the lungs or gills, the skin, the intestines, and the kidneys, are the outlets for excreted matters in all vertebrated animals.
1. There can be no doubt that the watery vapour and carbonic acid which are exhaled from the lungs, arc strictly excretions, although it is still doubted by some physiologists, whe ther the latter substance is truly exhaled, or rather formed at the lungs; on the latter sup position we should say, that the excretions of the lungs are water and carbon. It appears certain, from some experiments of Dr. Cordon, that no animal or saline matter escapes by this outlet. The total amount of loss by this excretion in twenty-four hours, in a middle-sized man, has been stated by Lavoisier and Seguin as aver aging about fifteen ounces ; and it must be re membered, that as we have good evidence of very considerable absorption at the lungs, the whole quantity of matter excreted must consi derably exceed this weight. Indeed, Mr. Dal ton estimates the exhalation of watery vapour only from the lungs at twenty-four ounces in the day. Some have estimated the quantity of carbon alone escaping in this way in the day at eleven ounces; but this estimate is probably exaggerated. It seems to be ascertained by the experiments of Dr. Edwards, of Despretz, and Collard de Martigny, that there is at times an obvious exhalation of' azote by the lungs; and Dr. Edwards expresses an opinion that there is probably, at all times, both an exhalation and absorption of that gas, but that these processes in general nearly compensate one another. According to Dr. Prouts views, re cently, though briefly, announced, we may, per haps, state the source and cause of the forma tion of the carbonic acid, and assign the use of the excretion of the water, which escapes by the lungs, with more precision. Ile supposes the acid to be evolved in the course of the circula tion, by that " process of reduction," by which the gelatin of the animal textures is formed from the albumen of the blood ; and the water to be given off chiefly from theweak albuminous matters of the chyle, and to be an essential part of the "process of completion," by which this is converted into the strong albumen (of the blood.* 2. The excretion by the skin is chiefly watery vapour; the escape of carbon, or carbonic acid, by this outlet appears to be to a very small amount, and to be very variable. In the sen sible perspiration or sweat there is an excess of lactic acid, a small quantity of the same animal and saline matters as are contained in the serum of the blood, and a little oily or fatty matter, probably from the sebaceous glands; the whole loss by this excretion in the human adult has been stated as averaging about thirty ounces in the day, but is evidently liable to very great variety. Many experiments prove that there is much less compensating absorption by this tex ture than by the lungs.