RATE OF THE RENAL EXCRETION. 773 



tirely, solid though in the Amphibia and Fishes, lower in the scale, 

 it is again fluid these bodies, though numerous, are remarkably small. 

 Moreover, the analogy of the minute vascular arrangements within the 

 kidney, with a portal system, is thought to be favored by the fact that, 

 in Reptiles, a branch from the hepatic portal vein is distributed to 

 the kidney. 



That some, at least, of the peculiar constituents of the urine, of 

 which urea, uric acid, creatin, and creatinin are the chief, exist pre- 

 formed in the blood, is certain ; for the two latter are found in it in 

 considerable quantity, urea in smaller quantity, and uric acid perhaps 

 only as an exceptional ingredient. It has been suggested that the 

 urea and the other crystalloids present in the inspissated blood may 

 pass, by swift dialysis, into the aqueous contents of the tubuli; but 

 though these substances, and urea especially, are highly dialyzable, 

 yet such a physical explanation of their separation from the blood, 

 would not account for their special appearance in the renal excretion, 

 rather than in any other, or, for the quantity eliminated from the 

 system in a given time, considering how minute is the normal propor- 

 tion of urea, and how much less that of uric acid, in the blood. It is, 

 indeed, impossible to deny that the spheroidal epithelial cells of the 

 tubes, have a special affinity for the proper urinary constituents; or, 

 and this may be of more moment in explaining their appearance in 

 the urine, that some of these substances are formed by the special 

 metamorphoses of other materials within these cells. In the former 

 case, the cells would merely select the urinary constituents from the 

 blood, and transmit them into the tubuli ; in the latter, they would, 

 in addition, be the seat of special chemical decompositions. In the 

 first case, for example, they might be supposed to separate pre-existing 

 urea from the blood, but, in the latter, to metamorphose creatin or 

 creatinin into urea. Indeed, after extirpation of the kidneys an 

 operation which animals will outlive a few days the blood is not found 

 to contain much urea, but to be rich in nitrogenous extractive matters, 

 which include creatin and creatinin. Moreover, if the ureters be tied, so 

 that the escape of the urine from the excreting structure of the kid- 

 neys is at first hindered, and at last prevented, urea is found in great 

 abundance in the blood, as if its formation had gone on in the sphe- 

 roidal epithelial cells, and it had been duly excreted, but then reab- 

 sorbed, or else had been absorbed from those cells directly into the 

 blood. 



It is further supposed that the smaller relative diameter of the effer- 

 ent vessels of the glomeruli, and the unusual blood pressure in the 

 renal arteries, may have an influence in the excretory work of the 

 renal apparatus. By some, it is thought, that a selective or meta- 

 morphic power of the cells, is indicated by the fact, that uric acid salts 

 are actually seen, in the cells lining the straight tubuli, in the kidneys 

 of Birds. Whilst the products of the decomposition of the tissues 

 are passing from the inspissated blood of the vascular plexus upon the 

 tubules, into the epithelial cells, some of the thin watery fluid within 

 the tubes is reabsorbed by those vessels, and thus the renal blood 

 partly regains its fluidity, whilst the urine becomes more concentrated. 



