THE KIDNEYS SECRETION OF URINE. 639 



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 tend 

 to remove pressure from the free surface of the vessels, -and so to encourage 

 the escape of their more fluid contents." 



841. There is a striking analogy between the mode in which the Tubuli 

 Uriniferi are supplied with Blood, for the purpose of elaborating their secre- 

 tion, and the plan on which the Hepatic circulation is carried on. The 

 secretion of the Liver is formed from blood conveyed to it by one large vessel, 

 the Vena Portse, which has collected it from the Venous capillaries of the 

 chylopoietic viscera, and which subdivides again to distribute it through the 

 liver. The secretion of the Kidney, in like manner, is elaborated from blood 

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

 Malpighian tufts; this blood is collected and conveyed to the proper secreting 

 surface, not by one large trunk (which would have been a very inconvenient 

 arrangement), but by a multitude of small ones, the efferent vessels of the 

 Malpighian bodies, which may be regarded as collectively representing the 

 Vena Portae, 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's finds support from the fact, 

 that in Reptiles (in which, as in Fishes, the Portal trunk receives the blood 

 from the whole posterior part of the body, and supplies the Kidneys as well 

 as the Liver), the efferent vessels of the Malpighian bodies which receive 

 their blood, as elsewhere, from the Renal Artery unite with the branches 

 of the Portal vein, 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 blood conveyed from the 

 chylopoietic viscera through the Vena Portse, and in part by the nutritive 

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

 Artery. 



842. The nature and purposes of the Urinary secretion, and the alterations 

 which it is liable to undergo in various conditions of the system, are much 

 better understood than are those of the Bile ; this is owing, in great part, to 

 the circumstance, that it may be readily collected in a state of purity ; and 

 that its ingredients are of such a nature, as to be easily and definitely sepa- 

 rated from each other by simple chemical means. There can be no doubt 

 that the chief purpose of this excretion, is to remove from the system the effete 

 azotized matters which the blood takes up in the course of the circulation, or 

 which may have been produced by changes occurring in itself. This is evi- 

 dent from the large proportion of Nitrogen which is contained in the solid 

 matter dissolved in it ; and from the crystalline form presented by this solid 

 matter when separated, a form which indicates that its state of combination 

 is such, as to prevent it from conducing to the nutrition of the system. The 

 injurious effects of the retention in the Blood, of the components of the Uri- 

 nary secretion, are fully demonstrated by the results of its cessation ; whether 

 this be' made to take place experimentally (as by tying the renal artery), or be 

 the consequence of a disordered condition of the kidney. Symptoms of great 

 disorder of the nervous centres, analogous to those produced by many narcotic 

 poisons, soon exhibit themselves ; and the patient dies comatose, if the secre- 

 tion be not restored. In such cases, Urea (the characteristic ingredient of the 

 urine) is found to have accumulated in the Blood ; and it may even be detected 

 by the smell, in the fluid effused into the Ventricles of the Brain. The con- 

 clusion which may be drawn from this circumstance, regarding the pre-exist- 



