THE KIDNEYS. SECRETION OF URINE. 599 



water which is passed off by the Kidneys, therefore, will depend 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 waste in the system, and upon 

 the quantity of surplus azotized aliment which has to be discharged through this 

 channel. The Kidney contains two very distinct provisions for these purposes. 

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

 ments by which the solid matter of the secretion is elaborated; whilst it can 

 scarcely be doubted that the office of the Corpora Malpighiana is to allow the 

 transudation of the superfluous fluid through the thin-walled and naked capilla- 

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

 (Op. cit., p. 75), "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 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 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 tend to remove pressure from the free surface of the vessels, and so 

 to encourage the escape of their more fluid contents." Here we see the essential 

 difference which exists between the vital agency concerned in the true Secreting 

 process, and the physical power which occasions fluid exhalation or transudation. 

 This difference is precisely the same as that which exists between the vital act 

 of selective absorption, and the physical operation of endosmose or imbibition. 

 By Imbibition and Transudation, certain fluids may pass through organic mem- 

 branes, in the dead as well as in the living body; and this passage depends 

 merely upon the physical condition of the part, in regard to the amount and the 

 nature of the fluid it .contains, and the permeability of its tissues. Not only 

 does water thus transude, but various substances that are held in complete solu- 

 tion in it, especially albuminous and saline matter : it is in this manner that the 

 Blood absorbs fluids from the digestive cavity ( 464), and pours out the serous 

 fluid which occupies the interspaces of the areolar tissue and the serous cavities. 

 The transudation of the watery portion of the blood is much increased by any 

 impediment to its flow through the vessels, and also by any causes that produce 

 a diminished resistance in their walls. 



637. 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 two 

 circumstances, 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 separated 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 evident from the large propor- 

 tion of Nitrogen contained in the solid matter dissolved in it; and from the 

 crystalline form presented by much of this solid matter when separated a form 

 which indicates that its state of combination is such, as to prevent it from con- 

 ducing to the nutrition of the system. The injurious effects of the retention of 

 the components of the Urinary secretion in the Blood are fully demonstrated 

 by the results of its cessation ; whether this be made to take place experiment- 

 ally (as by tying the renal artery), or be the consequence of a disordered condi- 



