410 THE ELIMINATION OF WASTE PRODUCTS. 



at present a matter of doubt ; it is not always the same, but appears not to 

 be fibrin. 



The most common and important abnormal constituents of urine are 

 albumin, giving rise to albuminuria, and sugar, giving rise to glycosuria or 

 diabetes. The soluble proteids, generally spoken of as " albumin," in the 

 urine differ in different cases. The exact determination of their nature is a 

 matter of some difficulty, since, as we have seen, we have in differentiating 

 the various proteids to trust largely to their behavior as regards precipita- 

 tion upon the addition of certain saline bodies ; and the presence of saline 

 bodies in the natural urine introduces complications. It would appear, 

 however, that the proteids usually present are serum-albumin and globulin ; 

 these are not, however, as a rule, if ever, present in the same relative pro- 

 portions as in blood-plasma ; and either the one or the other may be 

 present by itself. A form of albumose ( 189), called hemi-albumose, is 

 sometimes found, and, indeed, probably very many distinct kinds of pro- 

 teids are from time to time present. If egg-albumin be injected into the 

 blood it appears in the urine as egg-albumin, and peptone similarly injected 

 appears as peptone. 



The sugar which is found in the urine of diabetes is (indistinguish- 

 able from ordinary dextrose ; but whether it is absolutely identical with 

 that body, or whether the sugar in all cases of diabetic urine is ex- 

 actly the same in character, cannot, perhaps, as yet, be regarded as defi- 

 nitely settled. 



When blood is mingled with urine in the kidney and in the urinary 

 passages the constituents of the former are, of course, added to those of 

 the latter ; and when, as sometimes happens, chyle from the lacteals makes 

 its way into the kidneys, the urine contains the fats and other constituents 

 of chyle. Fats, however, may be present without the urine being distinctly 

 " chylous." 



Cholesterin, bile-acids, bile-pigments, and one or other of a large 

 number of bodies arising from a disordered metabolism of the body, such 

 as leucin, tyrosin, acetone (in cases of diabetes), oxalic acid, taurin, cystin, 

 and many others are also found more or less frequently ; some of these, 

 indeed, have been regarded as normal constituents. Besides these the urine 

 serves as the chief channel of elimination for various bodies, not proper 

 constituents of food, which may happen to have been taken into the sys- 

 tem. Thus various minerals, alkaloids, salts, pigmentary and odoriferous 

 matters, may be passed unchanged. Many substances thus occasionally 

 taken undergo, however, changes in passing through the body : the most 

 important of these, since the changes which they undergo throw light 

 on the metabolic processes of the body, will be considered in a succeeding 

 chapter. 



THE SECRETION OF URINE. 



345. The kidney consists of two parts, so distinct in structure that 

 it seems impossible to resist the conclusion that their functions are differ- 

 ent, and that the mechanism by which the urine secreted is of a double 

 kind. On the one hand, the tubuli uriniferi, with their characteristic epi- 

 thelium, seem obviously to be actively secreting structures comparable to 

 the secreting alveoli of the salivary and other glands. On the other hand, 

 the Malpighian capsules, with their glomeruli, are organs of a peculiar 

 nature, with an almost insignificant epithelium, and their structure irresist- 

 ibly suggests that they act rather as what may be called, in a general way, 

 a filtering than as a truly secreting mechanism. Hence has arisen the view 



