670 STRUCTURE OF KIDNEY. [BOOK n. 



to the cortex is sometimes spoken of as the boundary zone or 

 intermediate zone. 



395. Having thus traced out the devious and complex path 

 taken by a tubule we must study in more detail the special 

 characters of the several sections of its course. 



The Malpighian Capsule. Each tubule begins as we have said 

 in a globular expansion, having in man a diameter of about 200 //,, 

 the Malpighian capsule or end-capsule. The several capsules are 

 disposed for the most part in series of circles around the medullary 

 rays along their length, so that in radial sections of a kidney 

 they are seen in double radiating rows in the columns of cortical 

 substance between the medullary rays. Each capsule is essen- 

 tially a terminal globular expansion of a tubule, and consists, 

 like the tubule, of a distinct and conspicuous basement mem- 

 brane, having the ordinary characters of a basement membrane 

 ( 211), lined by an epithelium. At one pole of the sphere 

 the capsule is continued on into the tubule, its basement mem- 

 brane and its epithelium being continuous with the basement 

 membrane and the epithelium of the tubule; at the junction of 

 the two there is a marked constriction or neck. At the opposite 

 pole a short straight small artery (of whose relations we will speak 

 presently), vas afferens, runs into the capsule, driving before it and 

 inverting into the cavity of the capsule the basement membrane 

 and epithelium somewhat in the way that one might thrust one's 

 fist into and so invert at one part the wall of a large distended 

 elastic ball. Immediately upon its entrance into the capsule 

 the afferent artery divides into a number of branches. Each 

 branch further splits up into a number of capillary loops, the 

 returning limbs of the several loops joining, without lateral 

 anastomoses, to form a single vein-like vessel, vas efferens. The 

 whole lobulated bunch of branching and looped vessels has more 

 or less the appearance of a knot, and is called the glome- 

 rulus. The exact mode of division however differs in differ- 

 ent animals and apparently in different capsules in the same 

 kidney ; thus in the capsules nearer the medulla the glomeruli 

 are larger and more subdivided than in those nearer the surface. 

 The vas efferens starting from about the middle of the bunch 

 issues from the capsule side by side with the vas afferens, the 

 orifice formed by the inversion of the capsule being not wide 

 but narrow so as just to admit the entering and issuing vessels. 

 Hence the glomerulus hangs as it were into the cavity of the 

 capsule suspended by a narrow neck consisting of the afferent 

 and the efferent vessel, surrounded by the commencement of the 

 inverted portion of the wall of the capsule. When the blood vessels 

 are fully distended with blood the glomerulus fills the greater 

 part of the cavity of the capsule ; when they are constricted and 

 contain little blood, a space of some size is developed between the 

 surface of the glomerulus and the opposite wall of the capsule. 



