638 OF SECRETION. 



month, the kidneys consist of seven or eight lobes, the future pyramids; their excretory 

 ducts still terminate in the same canal, which receives those of the Wolffian bodies and of 

 the sexual organs; and this opens, with the rectum, into a sort of cloaca, or sinus urogeni- 

 talis, analogous to that which is permanent in the oviparous Vertebrata. The Kidneys are 

 at this time covered by the Supra-Renal Capsules, which are very large ; about the sixth, 

 month, however, these have decreased, whilst the kidneys have increased, so that their pro- 

 portional weight is as 1 to 4. At birth the weight of the Kidneys is about three times 

 that of the Supra-Renal Capsules; and they bear to the whole body the proportion of 1 to 

 80; in the adult, however, they are no more than 1 to 240. The Corpora Wolftiana are, 

 when at their greatest development, 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 einulgent vessels; and the lower into Spermatic vessels. The tabulated 

 appearance of the kidney gradually disappears ; partly in consequence of the condensation 

 of the areolar tissue, which connects the different parts; and partly through the develop- 

 ment of additional tubuli.inthe interstices. The Urinary Bladder is formed quite inde- 

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

 as a large coscum or diverticulum from the lower extremity of the alimentary canal (Chap, 

 xvii.). The part of the tube below this forms the Cloaca, or common termination of the 

 intestinal 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. 



840. 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 

 matter dissolved in it. The Kidneys, in fact, seem to form a kind of regu- 

 lating 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 ( 870) : 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 kid- 

 neys, 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 the channel. The Kid- 

 ney contains 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 



