STRUCTURE AND FUNCTION OF THE KIDNEYS. 359 



into the dilated extremity of the urinary tubule, which finally completely 

 invests it just as the pleura invests the lungs or the tunica vaginalis 

 the testicle. Thus the Malpighian capsule is lined by a parietal layer of 

 squamous cells and a visceral or reflected layer immediately covering the 

 vascular tuft (Fig. 245), and sometimes dipping down into its inter- 

 stices. This reflected layer of epithelium is readily seen in young sub- 

 jects, but cannot always be demonstrated in the adult. (See Figs. 248 

 and 249.) 



The vessels which enter the medullary layer break up into small ar- 

 terioles, which pass through the boundary layer, and proceed in a 

 straight course between the tubules of the papillary layer, giving off on 

 their way branches, which form a fine arterial mesh work around the 

 tubes, and ending in a similar plexus from which the venous radicles 

 arise. 



Besides the small afferent arteries of the Malpighian bodies, there 

 are, of course, others which are distributed in the ordinary manner, for 

 the nutrition of the different parts of the organ; and in the pyramids 

 between the tubes, there are numerous straight vessels, the vasa recta, 

 some of which are branches of vasa efferentia from Malpighian bodies, 

 and therefore comparable to the venous plexus around the tubules in the 

 cortical portion, while others arise directly as small branches of the renal 

 arteries. 



Between the tubes, vessels, etc., which make up the substance of the 

 kidney, there exists, in small quantity, a fine matrix of areolar tissue. 



Nerves. The nerves of the kidney are derived from the renal 

 plexus. 



The Ureters. The duct of each kidney, or ureter, is a tube about 

 the size of a goose-quill, and from twelve to sixteen inches in length, 

 which, continuous above with the pelvis of the kidney, ends below by 

 perforating obliquely the walls of the bladder, and opening on its inter- 

 nal surface. 



Structure. It is constructed of three principal coats (a) an outer, 

 tough, fibrous and elastic coat; (b) a middle muscular coat, of which 

 the fibres are unstriped, and arranged in three layers the fibres of the 

 central layer being circular, and those of the other two longitudinal in 

 direction; and (c) an internal mucous lining continuous with that of 

 the pelvis of the kidney above, and of the urinary bladder below. The 

 epithelium of all these parts (Fig. 250) is alike stratified and of a some- 

 what peculiar form; the cells on the free surface of the mucous mem- 

 brane being usually spheroidal or polyhedral with one or more spherical 

 or oval nuclei; while beneath these are pear-shaped cells, of which the 

 broad ends are directed towards the free surface, fitting in beneath the 

 cells of the first row, and the apices are prolonged into processes of vari- 



