CHAP. XXXIV.] OF THE SECRETION OF URINE. 495 



small and simple compared with those in the kidneys of animals 

 which pass the urinary constituents in solution in a large quantity of 

 water. There can be little doubt that the special function of the 

 vessels of the tuft is, to furnish the fluid portion of the urine while 

 the solid matter, composed of various organic constituents and 

 inorganic salts, is separated by the aid of the glandular epithelium 

 which lines the convoluted portion of the tubes. " It would, 

 indeed, be difficult to conceive a disposition of parts more calcu- 

 lated to favour the escape of water from the blood than that of the 

 Malpighian body. A large artery breaks up in a very direct 

 manner into a number of minute branches, each of which suddenly 

 opens into an assemblage of vessels of far greater aggregate 

 capacity than itself, and from which there is but one narrow exit 

 Hence must arise a very abrupt retardation in the velocity of the 

 current of blood. The vessels in which this delay occurs are 

 uncovered by any structure. They lie bare in a cell from which 

 there is but one outlet."* The arrangement of the convoluted por- 

 tion of the tubes is very similar to that of other secreting tubular 

 gland-structures. We have a delicate basement membrane in con- 

 tact with vessels upon one surface and having secreting epithelium 

 upon the other. The capillary net- work surrounding the urini- 

 ferous tubes, is the counterpart of that investing the tubes of the 

 testes ; and the epithelium is allied in structure to the best marked 

 examples of glandular epithelium, and there can be no doubt that 

 the function of these cells is such as we have described. There is 

 no reason for supposing that the cells of epithelium undergo rapid 

 decay and renovation; it appears more probable that they are 

 not being constantly shed, either in an entire or disintegrated state, 

 but that they have the power of selecting certain materials from 

 the blood, and afterwards giving them up without their destruction. 

 In the straight portion of the tubes the epithelium becomes thinner, 

 aud approaches more nearly to the pavement variety. It probably 

 serves principally as a protective covering, and takes no part what- 

 ever in the secretion of the urine. 



* Phil. Trans., 1842. 



