354 THE KIDNEYS AND THEIR SECRETION. 



merous straight vessels, the vasa recta, supposed by some ob- 

 servers to be branches of vasa efferentia from Malpighian 

 bodies, and therefore comparable to the venous plexus around 

 the tubules in the cortical portion, while others think that they 

 arise directly from small branches of the renal arteries. 



Between the tubes, vessels, &c., which make up the main 

 substance of the kidney, there exists in small quantity a fine 

 matrix of areolar tissue. 



The nerves of the kidney are derived from the renal plexus. 1 



Secretion of Urine. 



The separation from the blood of the solids in a state of so- 

 lution 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 chemi- 

 cal agency of the cells. 



The watery part of the urine is probably in part separated 

 by the same structures that secrete the solids, but the ingeni- 

 ous suggestion of Mr. Bowman that the water of the urine is 

 mainly strained off, so to speak, by the Malpighiau bodies, 

 from the blood which circulates in their capillary tufts, is ex- 

 ceedingly probable ; although if, as Kolliker and others main- 

 tain, there is an epithelial covering to these tufts or glomeruli, 

 it is very likely that the solids of the urine may be in part se- 

 creted 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 Malpighiau bodies for a 

 more simple draining off of water from the blood when re- 

 quired. 



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 por- 



1 For a more detailed account of the structure of the kidney and a 

 summary of the various opinions on the subject, the student may be 

 referred especially to Quain's Anatomy, 7th ed., and to a paper by 

 Dr. Reginald Southey, in vol. i of the St. Bartholomew's Hospital 

 Reports. 



