448 THE KIDNEYS AND THEIR SECRETION. 



Secretion of Urine. 



The separation from the blood of the solids in a state 

 of solution in the urine is probably effected, like other 

 secretions, by the agency of the gland cells, and equally 

 in all parts of the urine-tubes. The urea and uric acid, 

 and perhaps some of the other constituents existing ready 

 formed in the blood, may need only separation, that is 

 they may pass from the blood to the urine without further 

 elaboration ; but this is not the case with some of the 

 other principles of the urine, such as the acid phosphates 

 and the sulphates, for these salts do not exist as such in 

 the blood, and must be formed by the chemical agency of 

 the cells. 



The watery part of the urine is probably in part sepa- 

 rated by the same structures that secrete the solids, but 

 the ingenious suggestion of Mr. Bowman that the water 

 of the urine is mainly strained off, so to speak, by the 

 Malpighian bodies, from the blood which circulates in their 

 capillary tufts, is exceedingly probable ; although if, as 

 Kolliker and others maintain, there is an epithelial cover- 

 ing to these tufts or glomeruli, it is very likely that the 

 solids of the urine may be in part secreted here also. We 

 may, therefore, conclude that all parts of the tubular 

 system of the kidney take part in the secretion of the 

 urine as a whole, but that there is a provision also in the 

 arrangement of the vessels in the Malpighian bodies for a 

 more simple draining off of water from the blood when 

 required. 



The large size of the renal arteries and veins permits so 

 rapid a transit of the blood through the kidneys, that the 

 whole of the blood is purified by them. The secretion of 

 urine is rapid in comparison with other secretions, and as 

 each portion is secreted, it propels that which is already in 

 the tubes onwards into the pelvis of the kidney. Thence 

 through the ureter the urine passes into the bladder, into 



