SECRETION OF URINE. 531 



are no more than 1 to 240. The Corpora Wolffiana are, when at their greatest deve- 

 lopment, the most vascular parts of the body next to the liver; four or five branches 

 from the aorta are distributed to each, and two veins are returned from each to the vena 

 cava. The upper veins and their corresponding arteries are converted into the Renal or 

 emulgent vessels, and the lower into Spermatic vessels. The lobulated appearance of 

 the kidney gradually disappears; partly in consequence of the condensation of the areo- 

 lar tissue, which connects the different parts; and partly through the development of ad- 

 ditional tubuli in the interstices. The Urinary Bladder is formed quite independently of 

 the secreting apparatus, being a part of the allantois, which is first developed as a large 

 ccecum or diverticulum from the lower extremity of the alimentary canal (Chap. xiv.). 

 The part of the tube below this forms the Cloaca, or common termination of the intesti- 

 nal and vesical apparatus. The sides of this cloaca, however, gradually approach one 

 another, so as to form a transverse partition, which separates the Rectum from the Genito- 

 urinary canal; and the Urethra of the female is afterwards separated from the Vagina 

 by a similar process. 



668. The researches of Mr. Bowman on the structure of the Malpighian 

 bodies, and on the vascular apparatus of the Kidney, have thrown great light 

 upon the mode in which the Urinary secretion is elaborated. One of the 

 most remarkable circumstances attending this excretion, in the Mammalia 

 particularly, is the large but variable quantity of water, which is thus got rid 

 of, the amount of which bears no constant proportion to that of the solid mat- 

 ter dissolved in it. The Kidneys, in fact, seern to form a kind of regulating 

 valve, by which the quantity of water in the system is kept to its proper 

 amount. The Exhalation from the Skin, which is the other principal means 

 of removing the superfluous liquid from the blood, is liable to great variations, 

 from the temperature of the air around ( 731) : hence, if there were not some 

 other means of adjusting the quantity of fluid in the Blood-vessels, it would be 

 liable to continual and very injurious variation. This important function is 

 performed by the Kidneys ; which allow such a quantity of water to pass into 

 the urinary tubes as may keep the pressure within the vessels nearly at a 

 uniform standard. The quantity of 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 con- 

 veyed 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 con- 

 tains two very distinct provisions for these purposes. The cells lining the 

 Tubuli Uriniferi are probably here, as elsewhere, the instruments 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 capillaries of which 

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



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

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



