434 THE SOLIDS. 



The tubes themselves, and very rarely their globular ter- 

 minations, may be injected from the ureter : this is accom- 

 plished more readily in the kidneys of some animals, as the 

 horse, than in those of man. 



A complete Malpighian body, then, consists of the globular 

 enlargement of the tube, over which is spread the Malpighian 

 plexus, formed by branches of the renal artery and portal vein ; 

 this plexus does not consist entirely of capillary meshes, but 

 of vessels of different diameters ; the artery divides and sub- 

 divides : its terminal branches are, however, capillary in size, 

 but, in place of forming distinct meshes, follow a serpentine 

 and convoluted course. (See Plate LX. Jig. 2.) 



Mr. Bowman conceived, as already noticed, that the Mal- 

 pighian plexus was situated within the globular enlargement 

 of the uriniferous tube ; and, reflecting on the remarkable 

 structure of the Malpighian bodies, and on their singular con- 

 nection with the tubes, was led to consider that the tubes 

 and their plexus of capillaries are the parts concerned in the 

 secretion of that portion of the urine to which its character- 

 istic properties are due (the urea, lithic acid, &c.), while the 

 Malpighian bodies are an apparatus destined to separate from 

 the blood the watery portion of the urine. 



" It would indeed be difficult," Mr. Bowman writes, " to conceive a 

 disposition of parts more calculated 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 of 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 orifice of the tube. This orifice is encircled by cilia, in active 

 motion,, directing a current towards the tube. These exquisite organs 

 must not only serve to carry forward the fluid already in the cell, and 

 in which the vascular tuft is bathed, but must tend to remove pressure 

 from the free surface of the vessels, and so to encourage the escape of 

 their more fluid contents. Why is so wonderful an apparatus placed at 

 the extremity of each uriniferous tube, if not to furnish water, to aid in 

 the separation and solution of the urinous products from the epithelium 

 of the tube?" P. 75. 



