THE KIDNEYS SECRETION OF URINE. 511 



in chemical analysis, and the evidence is at present in an extremely unsatis- 

 factory state. As regards the mode of origin of urea, its composition shows 

 clearly that it proceeds from the disintegration of the albuminous or nitro- 

 genous constituents of the tissues and blood. It was at one time consid- 

 ered that it was generated entirely at the expense of the muscular tissue; 

 but as no one has yet succeeded in extracting urea from muscle, and as 

 muscular exertion, even when violent and protracted, causes little or no 

 increase in the amount eliminated, and as the substances that we know result 

 from the disintegration of muscular tissue, as kreatin and kreatiuin, are either 

 not at all, or not easily, converted into urea, and even when ingested are 

 eliminated unchanged, without causing any augmentation in the amount of 

 urea, whilst a large increase quickly follows the ingestiou of nitrogenous food 

 when the body is at rest, there seem to be good grounds for admitting that 

 only a portion of the urea discharged can proceed from that disintegration 

 to which the muscles, in common with all other animal structures of high 

 organization, are liable, and that another and much larger portion is derived 

 from the decomposition and disintegration of the albuminous compounds that 

 are constantly being consumed in maintaining the activity of the great gland- 

 ular and cellular organs of the body, 1 the ultimate results of the oxidation 

 urea, carbonic acid, and water being the same. The question of the 

 place of origin of the urea is still more difficult! to answer. Up to a very 

 recent period it was universally held that the kidneys constituted a kind of 

 filter by which certain products of the disintegration of the tissues were 

 separated from the blood and discharged from the body. M. Picard's experi- 

 ments, which showed that urea is uniformly present in the blood (2-4 parts 

 in 10,000), and in larger proportion in the renal arterial than in venous 

 blood, and those of Grehant, 2 showing that after ligature of the ureters the 

 amount of urea in renal venous blood becomes exactly equal to that in renal 

 arterial blood, have been commonly referred to in support of this view, the 

 small quantity present being attributed to the care with which the system 

 was freed from this excretory product. Lately, however, an attempt has 

 been made to prove that urea is formed from the secondary products of the 

 disintegration of the tissues, such as kreatin, kreatinin, uric acid, hypoxan- 

 thin, and taurin, in and by the kidney itself, upon the following grounds: 3 

 First, that extirpation of the kidneys is followed by only a trifling increase 

 in the proportion of urea present in the blood, though the proportion of 

 kreatiu and other lower stages of the oxidation of nitrogenous compounds is 

 augmented both in the blood and muscles; secondly, that after ligature of the 

 ureters, the renal organs remaining intact, there is considerable increase in. 

 the amount of urea contained in the blood ; thirdly, that the ingestion of 

 kreatiu with the food produces an increase of the excretion of kreatinin and 

 urea; fourthly, that on rubbing down kreatin with the substance of the 

 kidney, it is rapidly converted into urea ; and lastly, that there is reason for 

 believing that the cells covering the glomeruli exercise a secreting function, 



1 On this view the " luxus consumption" of Voit and others, which was considered 

 to consist in the oxidation of the nitrogenous constituents of the food in the blood 

 previous to their conversion into tissue, has no existence. 



2 See abstract of Memoir in Humphry and Turner's Journal of Anat. and Phys., 

 1871, p. 216. 



3 See Munk, Deutsche Klinik, 1862, p. 299; Oppler, Virchow's Archiv, 1861, Bd. 

 xxi, p. 260; Subbotin, Henle and Meissner's Zeitschrift, Bd. xxviii, 1866, p. 114; 

 and Zalesky, Untersuchungen uber den Ura^mischen Process. Schultzen and Nencki 

 (Zeits. f. Biologie, 1872, Bd. vii, p. 124) believe that some of the intermediate stages 

 between Albumen and Urea are represented by Glycol, Leucin, and Tvrosin. But 

 Kussner has clearly shown that Tyrosin cannot be regarded as an antecedent of Urea 

 (Centralblatt, 1875, p. 216). 



