82 CHEMICAL COMPONENTS OF THE HUMAN BODY. 



53. That Urea is to be regarded as one of the most important products of the 

 disintegration of the azotized tissues of the body, cannot now be doubted for a 

 moment. It may be produced by the decomposition of various nitrogenous sub- 

 stances, both natural and artificial; and it is a fact of the greatest physiological 

 interest (as will be presently apparent), that it may be obtained from creatine by 

 the action of baryta-water or alkalies. In inquiring into the precise mode of its 

 production, we must trace it backwards (so to speak) from the Urine, of which 

 in Man it is the characteristic ingredient. There is now abundant evidence, that 

 urea is not generated (as formerly supposed) in the act of secretion, but that it 

 exists preformed in the blood ; for not only does it accumulate largely in the 

 circulating fluid, when its elimination by the normal outlet is checked, but it 

 presents itself, though in extremely small amount, in healthy blood, as has been 

 determined by Strahl and Lieberkiihn, Garrod, and (still more decisively) by 

 Lehmann. That its proportion in the circulating fluid never arises to an amount 

 that would render it easily detectable, notwithstanding that it is continually 

 either being generated within the vessels, or being absorbed into them from 

 without, is readily accounted for by the avidity (so to speak) with which the 

 kidney seizes upon even the smallest proportion of this substance, and eliminates 

 it from the nutritive fluid. The purpose with which this organ has been enabled 

 thus to act, becomes obvious enough when it is remembered that Urea is a sub- 

 stance whose accumulation in the blood is most pernicious ; as is seen in various 

 forms of disease, of which the most severe symptoms, and not unfrequently the 

 fatal termination itself, are directly attributable to this mode of poisoning. 

 Moreover, it has been shown, by the calculations of Lehmann, that it would 

 require the whole urea generated in the system to accumulate in the blood for 

 at least an -hour (so as to impregnate it with l-24th part of the total quantity 

 passed in one day) before it would amount to such a percentage as can be cer- 

 tainly detected in that fluid. Even if it could never be recognized in healthy 

 blood, therefore, no argument would be thence furnished against the doctrine 

 that it is present as such in the blood before it passes to the kidneys. This 

 doctrine is confirmed by the fact, that Urea may be discovered in various fluids 

 separated from the blood ; and this not only when the normal elimination is 

 checked, but also in the usual state of health. Thus it has been found by 

 Millon and Wohler in the vitreous and aqueous humors of the eye, by Rees in 

 milk and liquor amnii, and by Landerer in the sweat ; and when the renal 

 arteries have been tied, or the kidneys have been removed, or their secreting 

 action has been interfered with by injury to their nerves or by disease, Urea 

 has been detected in almost every secretion and fluid exudation that has been 

 drawn or poured forth from the circulating current. The question now arises, 

 whether the Urea thus contained within the blood-vessels is generated there, or 

 whether it is taken up from extraneous sources, to be conveyed by the current 

 of the circulation to the excretory outlets. Now in favor of the first of these 

 opinions it may be urged, that, as will appear hereafter (CHAP, xn., SECT. 3), 

 Urea is not formed only from the disintegration of the living tissues, but that it is 

 also generated at the expense of superfluous alimentary materials, which, having 

 been introduced into the blood, and not being required to supply the wants of 

 the system, pass into decomposition without ever having been converted into 

 tissue. Such being the case, a part at least of the Urea that is eliminated from 

 the blood may be considered as having been generated within the vessels ; since 

 it is very seldom that of the supply of nitrogenous aliment ingested by man, 

 there is not some portion that may be considered superfluous. Now of that 

 portion which has its origin in the disintegration of the tissues, the greater part 

 may be set down to metamorphosis of the muscular substance, since it is found 

 that muscular exercise has a special power of augmenting the quantity of urea 



