CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 635 



space than the two together ; hence the medullary substance 

 becomes less as it converges towards the hilus. The medulla is 

 moreover divided into a number (varying in different animals, 

 being one in the rabbit and the rat, and about ten or twelve 

 in man) of masses, each of which, since it diminishes in bulk 

 towards the hilus, has the form of a pyramid, pyramid of 

 Mulpiglri, with its apex directed radially towards the hilus and 

 its base resting on and becoming confused with the cortex. 



The ureter or main duct of the kidney when traced to the 

 kidney, is found to expand at the hilus into a funnel-shaped cavity, 

 the pelvis, which divides or branches somewhat irregularly into a 

 number (equal to that of the pyramids) of short broad tubes, 

 coli/ces, somewhat in the way that the hand of a glove divides 

 into the fingers, but more irregularly. Into each calyx the 

 summit of a corresponding pyramid projects for some little way in 

 the form of a nipple, or papilla, the epithelium lining the calyx 

 being thus continuous with, and as it were reflected to form 

 the epithelium covering the projecting nipple of the pyramid. 

 The straight tubules forming as we have seen the pyramid, 

 though numerous at its base, become by repeated junctions fewer 

 and larger, and finally form a number (in man about a score) 

 of relatively wide tubules which open into the calyx at or near 

 the very summit of the nipple ; here the epithelium lining the 

 tubules becomes continuous with the epithelium covering the 

 papilla. 



Hence in a radial section of human kidney (one taken in the 

 long axis being preferable) the whole outer portion of the organ, 

 all round except at the hilus, will be seen to be occupied by the 

 fairly uniform cortex which, being composed as we have said 

 mainly of tubes twisting in all directions, presents on section to 

 the naked eye a granular aspect. From this cortex will be seen 

 converging towards the hilus a certain number of pyramids, each 

 of which since it is mainly composed of radiating straight tubules, 

 and since the minute blood vessels ramifying in it have a similar 

 radiating straight course, will present to the naked eye a more 

 or less marked radiating grain or striation. The apex of each 

 pyramid where the section has passed through the apex, will 

 be seen projecting into its appropriate calyx, the calyces will be 

 seen uniting to form the pelvis, and provided that the plane of 

 section has passed through the mouth of the ureter, the pelvis will 

 be seen narrowing into the ureter. The section may of course 

 have missed the ureter ; it is also very likely to have cut one or 

 other of the pyramids higher up than the attachment of the calyx, 

 in \vhich case of course the projection of the papilla of the pyramid 

 into its calyx is not seen. 



The pyramids are separated from each other laterally, above 

 the attachment of their respective calyces, partly by a small quan- 

 tity of cortical substance which creeps down their sides towards 



