CHAP, iv.] RENAL SECRETION. 409 



walls, and so allowed the passage through them into the interior of 

 the Malpighian capsules of the natural proteids of the blood, which 

 in a normal condition of the capillaries cannot effect such a 

 passage. The injury however was temporary only; in a short time 

 the capillary walls were restored to health and the urine ceased to 

 be albuminous. 



We may further quote as shewing the peculiar nature of 

 the nitration that ligature of the renal veins arrests the secretion 

 of urine. Apparently the effect which it should produce by 

 increasing the pressure in the glomeruli is more than counter- 

 balanced by other influences. Upon removal of the ligatures, the 

 urine is usually albuminous, shewing that in the interval the 

 glomeruli have become changed. 



One consideration, of quite secondary importance in the glands 

 which have been previously studied, acquires great prominence 

 when the kidney is being studied. In studying the pancreas and 

 gastric glands, we concluded without much discussion that the 

 zymogen and pepsinogen were formed in the epithelium cells ; for 

 no great manufacture of these substances is going on in other parts 

 of the body. The kidney however is emphatically an excreting 

 organ : its great function is to get rid of substances produced by 

 the activity of other tissues ; its work is not to form but to eject. 

 There can be no doubt, to put forward a strong instance, that with 

 regard to urea it would be absurd to suppose that the whole series 

 of changes from the proteid condition to the urea stage is carried 

 on by the kidney. But there still remains the question, Are any 

 of the stages carried on in the kidney, and if so, what ? Is the 

 secreting activity of the renal epithelium confined to picking out the 

 already formed urea from the blood ? Or does the secreting cell of 

 the tubule receive from the blood some antecedent of urea, and in 

 the laboratory of its protoplasm convert that antecedent of urea 

 into urea itself ? and if so, what is that antecedent which comes to 

 the kidney in the blood of the renal artery ? And so with many 

 other of the urinary constituents. 



In order to complete our study of renal activity, this question 

 ought to be considered now ; but for many reasons it will be more 

 convenient to defer the matter to the succeeding chapter, in which 

 we deal with the metabolic events of the body in general. 



