640 STRUCTURE OF KIDNEY. [BOOK n. 



The epithelium lining the wall of the capsule consists of flat 

 polygonal nucleated cells which have almost an epithelioid 

 character. Indeed they are seen with difficulty and are best 

 brought into view by the silver nitrate method. These cells rest 

 on a basement membrane which as we have said is distinct, and 

 in optical or other section presents a sharp outline. 



The basement membrane over the glomerulus cannot be so 

 readily distinguished. It appears to be completely fused with the 

 wall of the capillary loops, which like other capillaries consist of a 

 homogeneous membrane of nucleated epithelioid plates cemented 

 together. The epithelium covering the glomerulus, which follows 

 the inequalities of the surface, forming a covering for and dipping 

 down Between the groups of capillary loops, and hence is in close 

 contact with the blood vessels, is said to differ from the epithelium 

 lining the wall of the capsule inasmuch as the cells do not so 

 closely resemble epithelioid plates, but are flattened cells, often 

 irregular in form, each with a transparent or faintly granular 

 cell-substance and rounded nucleus; they are distinctly cubical 

 in the new-born animal but become flatter in the adult. Thus 

 each of the capillary loops of the glomerulus appears to project 

 into the cavity of the capsule in such a way that the blood 

 in the vessel is separated from the cavity of the capsule, and so 

 from the lumen of the tubule, first by a thin film composed of 

 the capillary wall (with which the basement membrane of the 

 inverted portion of the capsule has become fused), and next by 

 an epithelium cell of somewhat peculiar nature. As we shall 

 presently see some of the problems concerning the secretion of 

 urine turn on the nature of the processes carried out by this 

 film covered with this epithelium. 



Each capsule is surrounded by a small quantity of connective- 

 tissue which, very scarce in the kidney generally, is more obvious 

 here than elsewhere. A small amount of connective-tissue also 

 surrounds the afferent and the efferent vessel of the glomerulus, 

 but a minimum of this tissue is carried into the capsule with 

 the glomerulus. Indeed the presence of connective-tissue to form 

 a middle to or a support of a loop or even in the depths of the 

 glomerulus, cannot be definitely demonstrated. Hence, though 

 we have reason to think that lymphatics exist in the tissue 

 around the capsule as elsewhere in the kidney, it has been 

 maintained that lymphatics are absent in the glomerulus between 

 the blood vessels. In at all events the peripheral portion of each 

 capillary loop, covered as it seems to be closely by epithelium, 

 the only exit of material through the capillary wall leads direct 

 through the epithelium into the cavity of the capsule. 



The capsule is continued on into a tubule by a short con- 

 stricted portion or neck; and here the epithelium suddenly 

 changes in character and puts on the features which we are now 

 about to describe. 



