494 OF SECRETION AND EXCRETION. 



between the mode in which the tubuli uriniferi are supplied with blood, for the 

 purpose of elaborating their secretion, and the plan on which the hepatic circu- 

 lation is carried on. For as the secretion of the Liver is formed from blood 

 conveyed to it by one large vessel, the portal vein, which has collected it from 

 the venous capillaries of the chylopoietic viscera, and which subdivides again 

 to distribute it through the liver, so the secretion of the Kidney is elaborated 

 from blood which has already passed through one set of capillary vessels, those 

 of the Malpighiau tufts; this blood is collected and conveyed to the proper 

 secreting surface, however, not by one large trunk (which would have been a 

 very inconvenient arrangement), but by a multitude of small ones, the effer- 

 ent vessels of the Malpighian bodies, which may be regarded as collectively 

 representing the portal vein, since they convey the blood from the systemic 

 to the secreting capillaries. Hence the Kidney may be said to have a portal 

 system within itself. This ingenious view of Mr. Bowman finds support from 

 the fact, that in Reptiles the efferent vessels of the Malpighiau bodies (which 

 receive their blood, as elsewhere, from the renal artery) unite with the renal 

 branches of the Vena Portse, to form the secreting plexus around the tubuli 

 uriniferi. Here, therefore, the blood of the secreting plexus has a double 

 source, the vessels which supply it receiving their blood in part from the 

 capillaries of the organ itself, and in part from those of viscera external to 

 it; just as, in the Liver, the secreting plexus is supplied in part by the nu- 

 tritive capillaries of the organ itself, which receive their blood from the 

 hepatic artery, and in part by the blood conveyed from the chylopoietic 

 viscera through the Vena Portse. 



404. These admirable researches of Mr. Bowman on the structure of the 

 Malpighiau bodies, and on the vascular apparatus of the Kidney, have 

 thrown great light upon the mode in which the Urinary secretion is elabo- 

 rated. One of the most remarkable circumstances attending this excretion, 

 in the Mammalia particularly, is the large but variable quantity of water, 

 which is thus eliminated, the amount of which bears no constant propor- 

 tion to that of the solid matter dissolved in it. The quantity of water which 

 is passed off by the Kidneys depends in part upon that exhaled by the Skin, 

 being greatest when this is least, and vice versa; but the quantity of solid 

 matter to be conveyed away in the secretion has little to do with this, being 

 dependent upon the amount of ivaste in the system, and upon the quantity 

 of surplus azotized aliment which has to be discharged through this chan- 

 nel. The Kidney- contains two very distinct provisions for these purposes. 

 The cells lining the tubuli uriniferi are probably here, as elsewhere, the in- 

 struments by which the solid matter of the secretion is eliminated ; whilst it 

 can scarcely be doubted, that the chief office of the Corpora Malpighiana is 

 to allow the transudatiou of the superfluous fluid through the thin-walled 

 capillaries of which they are composed. "It would indeed," Mr. Bowman 

 remarks, 1 "be difficult to conceive a disposition of parts more calculated to 

 favor 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 nar- 

 row exit. Hence must arise a very abrupt retardation in the velocity of the 

 current of blood." The vessels in which this delay occurs, lie in a capsule, 

 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 which 

 is already in the cell, and in which the vascular tuft is bathed ; but must 



1 Loc. cit., p. 75. 



