EXCRETION. 



149 



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, are 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. Dai- 

 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. Prout's 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. He 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 the weak 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 



* See Bridgewater Treatise, p. 524. 



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. 



3. The excretions by the bowels are, properly 

 speaking, only those parts of the alvine evacua- 

 tions, which are secreted within the body itself, 

 and mixed with the residue of the food. It is 

 probable that part of the secretions from all 

 parts of the primae viae are thus excreted, but 

 the only one of which it has been ascertained 

 that it is, in part at least, destined necessarily 

 for excretion, is the bile. It is certain that the 

 peculiar animal matter of this secretion, (re- 

 garded by some as of pretty simple and by 

 others as of very complicated composition) is 

 never found in the healthy state in the lacteal 

 vessels or thoracic duct that it is found in full 

 quantity along with the residue of the aliments 

 in the lower intestines, that it is increased in 

 quantity when the excretion of urine is sup- 

 pressed in animals by extirpation of the kid- 

 neys ; and again, that when this secretion is sup- 

 pressed, the urine is increased and altered ; and 

 we can therefore have no difficulty about regard- 

 ing this part of the bile as strictly an excretion, 

 notwithstanding that we have good evidence, 

 that at least the alkali of the bile is of use in 

 the digestion and assimilation of the food. Of 

 the quantity of matter strictly excreted from the 

 intestines in the day it must of course be very 

 difficult to judge. The chemical elements that 

 escape in the biliary matter must be chiefly 

 carbon and hydrogen. 



4. The urine is the most complex of the ex- 

 cretions, particularly as to saline impregnation, 

 containing not only the salts which are detected 

 in the blood, but a portion of every earthy and 

 saline matter that can be found in any part of 

 the body, besides the peculiar and highly azo- 

 tised animal matters, lithic acid and urea. The 

 average quantity of urine passed in twenty-four 

 hours may be about forty ounces, but is very 

 liable to variation, particularly by temperature, 

 being generally greater, as the excretion by the 

 skin is less. The quantity of solid matter, 

 animal, earthy, and saline, that passes off in 

 this way has been stated at about fifteen drachms 

 on an average, and is evidently much less liable to 

 change, the density of urine, in the healthy state, 

 always diminishing as its quantity increases, and 

 vice versa. The milk, and the semen, although 

 destined to no useful office in the system in 

 which they are formed, are rather to be called 

 recrementitious secretions than excretions. Yet 

 the former has this property in common with 

 excretions, that its retention within the body, 

 when the conditions of its formation exist, i& 



