528 



OF SECRETION. 



Fig. 152. 



Fig. 153. 



Portion of the Kidney of a new-born infant; A, natural Supposed coecal extremity of one of the 



size ; 1, 1, corpora Malpighiana, as dispersed points in the tubuli uriniferi, from the kidney of an 



cortical substance; 2, 2, papilla; B, a smaller part magni- adult; showing its tesselated epithelium 



fied; 1, 1, corpora Malpighiana; 2, 2, tubuli uriniferi (After magnified 250 diameters. (After Wagner.) 

 Wagner.) 



the tube is continuous with this capsular dilatation (Fig. 155,6'); being there more trans- 

 parent, and furnished with cilia (as shown at 6"), which in the Frog may be seen, for 

 many hours after death, in very active motion, directing a current down the tube. Fur- 

 ther within the capsule, the Epithelium is excessively delicate, and even in many cases 

 absent. The Renal Artery, on entering the Kidney, divides itself into minute twigs, 

 which are the afferent vessels of the Malpighian tufts (Fig. 156). After it has pierced the 

 capsule, the twig dilates ; and suddenly divides and subdivides itself into several minute 

 branches, terminating in convoluted capillaries, which are collected in the form of a ball; 

 and from the interior of the ball, the solitary efferent vessel arises, which passes out of 

 the capsule by the side of the single afferent vessel. This ball lies loose and bare in the 

 capsule, being attached to it only by its afferent and efferent vessels (Fig. 156, m); and 

 every vessel composing it is bare and uncovered, an arrangement of which the economy 

 presents no other example. The efferent vessels, on leaving the Malpighian bodies, sepa- 

 rately enter the plexus of capillaries surrounding the tubuli uriniferi, and supply that 

 plexus with blood ; from this plexus the renal vein arises. In Mr. Bowman's opinion, all 

 the free extremities of the tubuli uriniferi thus include Corpora Malpighiana; and the ap- 

 pearance of coecal terminations, such as those represented in Fig. 153, and at a and c Fig. 

 154, he regards as an optical illusion, caused by a change in the direction of the tubuli, 

 which occasions them to dip away suddenly from the observer. 



c. The Embryological Development of the Urinary organs in Vertebrated animals is a 

 subject of peculiar interest, owing to the correspondence which may be traced between 

 the transitory forms they present in the higher classes, and their permanent condition in 

 the lower. In this respect there is an evident analogy with the Respiratory system ; and 

 it may be remarked that the analogy does not cease here. Both the Urinary and the Pul- 

 monary organs are destined to excrete the products of decomposition, united in their sim- 

 plest forms of combination ; carbon and oxygen being thrown off from the lungs or gills, 

 in the form of carbonic acid ; and carbon and nitrogen from the kidneys, in the form of 

 cyanogen, which unites with oxygen to produce cyanic acid, and this combines with am- 

 monia (a compound of nitrogen and hydrogen) to form urea, the characteristic element of 

 the urinary secretion. Both organs, moreover, have the important function of getting rid 

 of the superfluous fluid of the body. Again, it is an interesting fact that, in the Holo- 

 thuria and other animals of its type, the Respiration is performed by the introduction of 

 water through a system of branching tubes, that extend from the cloaca into the interior 

 of the body; this system of tubes, considered in regard to its structure and position, is 

 evidently analogous to the Urinary apparatus of higher animals. The first appearance 

 of any thing resembling a Urinary apparatus in the Chick, is seen on the second half of 

 the third day. The form at that time presented by it is that of a long canal, extending on 

 each side of the Spinal Column from the region of the heart towards the Allantois ; and 

 the sides of this present a series of elevations and depressions, indicative of the com- 

 mencing development of coeca. On the fourth day, the Corpora Wolffiana, as they are 

 then termed, are distinctly recognized, as composed of a series of coecal appendages, 

 which are attached along the whole course of the first-mentioned canal, opening into its 

 outer side. On the fifth day these appendages are convoluted, and the body which they 

 form acquires increased breadth and thickness. They evidently then possess a secreting 

 function ; and the fluid which they separate is poured by the long straight canal into the 



