REN. 



255 



these respects the Malpighian bodies differ 

 from the secreting parts of true glands. 1 . The 

 Malpighian bodies comprise but a small part 

 of the inner surface of the kidney, there being 

 but one to each tortuous tube. 2. The epi- 

 thelium immediately changes its characters, 

 as the tube expands to embrace the tuft of 

 vessels. From being opaque and minutely 

 mottled, it becomes transparent, and assumes 

 a definite outline ; from being bald, it becomes 

 covered with cilia (at least in reptiles, and 

 probably in all classes) ; and in many cases it 

 appears to cease entirely a short way within 

 the neck of the Malpighian capsule. 3. The 

 blood-vessels, instead of being on the deep 

 surface of the membrane, pass through it and 

 form a tuft on its free surface. Instead of 

 the free anastomoses elsewhere observed, 

 neighbouring tufts never communicate, and 

 even the branchlets of the same tuft remain 

 quite isolated from one another. 



" Thus the Malpighian bodies are as unlike, 

 as the tubes passing from them are like, the 

 membrane which, in other glands, secerns its 

 several characteristic products from the blood. 

 To these bodies, therefore, some other and 

 distinct function is with the highest probabi- 

 lity to be attributed. 



" When the Malpighian bodies were con- 

 sidered merely as convoluted vessels, without 

 any connection with the uriniferous tubes, no 

 other office could be assigned them than that 

 of delaying the blood in its course to the 

 capillaries of the tubes, and the object of 

 this it was impossible to ascertain. Now, 

 however, that it is proved that each one 

 is situated at the remotest extremity of a 

 tube, that the tufts of vessels are a dis- 

 tinct system of capillaries inserted into the 

 interior of the tube, surrounded by a capsule 

 formed by its membrane, and closed every- 

 where except at the orifice of the tube, it is 

 evident that conjectures on their use may be 

 framed with greater plausibility. 



" The peculiar arrangement of the vessels 

 in the Malpighian tufts is clearly designed to 

 produce a retardation in the flow of the blood 

 through them ; and the insertion of the tuft 

 into the extremity of the tube, is a plain indi- 

 cation that this delay is subservient in a direct 

 manner to some part of the secretive process. 



" It now becomes interesting to inquire, in 

 what respect the secretion of the kidney dif- 

 fers from that of all other glands, that so 

 anomalous an apparatus should be appended 

 to its secerning tubes. The difference seems 

 obviously to lie in the quantity of aqueous 

 particles contained in it ; for how peculiar 

 soever to the kidney the proximate principles 

 of the urine may be, they are not more so 

 than those of other glands to the organs 

 which furnish them. 



" This abundance of water is apparently in- 

 tended to serve chiefly as a menstruum for 

 the proximate principles and salts which this 

 secretion contains, and which, speaking gene- 

 rally, are far less soluble than those of any 

 other animal product. This is so true, that 

 it is common for healthy urine to deposit 



some part of its dissolved contents on cool- 

 ing. 



" If this view of the share taken by the 

 water be correct, we must suppose that fluid 

 to be separated either at any point of the 

 secreting surface along with the proximate 

 principles, as has hitherto been imagined, or 

 else in such a situation that it may at once 

 freely irrigate the whole extent of the secern- 

 ing membrane. Analogy lends no counte- 

 nance to the former supposition ; while to the 

 latter, the singular position and all the details 

 of the structure of the Malpighian bodies, give 

 strong credibility. 



" It would indeed be difficult 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 num- 

 ber of minute branches, each of which sud- 

 denly 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 un- 

 covered 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 pres- 

 sure from the free surface of the vessels, and 

 so to encourage the escape of their more 

 fluid contents. Why is so wonderful an ap- 

 paratus placed at the extremity of each urini- 

 ferous tube, if not to furnish water to aid in 

 the separation and solution of the urinous 

 products from the epithelium of the tube?" 



There is nothing which appears to afford 

 greater support to Mr. Bowman's theory than 

 the structure of the kidney of the boa, when 

 considered in connexion with the fact that the 

 urine in this animal is excreted in an almost 

 solid form. It will be remembered-j- that the 

 greater part of the blood supplied to the kid- 

 ney of the boa is derived from a vein which 

 comes from the posterior part of the body ; 

 this vein forms the plexus which surrounds 

 the uriniferous tubes, and from which, accord- 

 ing to Mr, Bowman, the solids of the urine 

 are excreted. The renal artery, which is com- 

 paratively of small size, is distributed to the 

 Malpighian bodies, as in the higher animals, 

 and the efferent vessel joins the portal vein. 

 The solid urine of the serpent seems a neces- 

 sary consequence of the peculiar distribution 

 of the blood-vessels ; the small Malpighian 

 bodies pour out a scanty stream of water 

 sufficient only to carry through the tubes the 

 large quantities of solid matter which the 

 more numerous and larger vessels distributed 

 on the outer surface of the tubes are continu- 

 ally supplying. 



* With reference to this point, vide ante, p. 248-9. 

 f Vide ante, p. 250. 



