THE KIDNEYS 



171 



process of excretion goes on, and that the watery fluid (the urine) 

 there separated passes through the minute tubes of the pyramids 

 into the pelvis, whence it is conveyed to the bladder by the ureter. 

 This is actually the case ; but in order to understand how this is 

 brought about it will be necessary to give a more detailed exami- 

 nation, aided by the microscope. 



If we examine a thin prepared section of the kidney under a powerful 

 microscope, we notice that the straight tubes of the pyramids (the uriniferous 



Fig. 158. Longitudinal Section of 

 the Human Kidney. One-half the 

 natural size. 



a, cortical substance ; &, pyramids broad 

 portions ; c, calyces of the pelvis laid open ; 

 c 1 , calyx, unopened ; d, summits of the 

 pyramids projecting intocalices ; e, narrow 

 parts of pyramids ; /, pelvis ; u, ureter ; 

 A, hilus ; s, extension of the hilus. 



Fig. 159. Diagram show- 

 ing the relation between 

 the Uriniferous Tubes and 

 the Blood-vessels. 



a, small branch of the renal artery ; 

 a 1 , smaller branch passing into a 

 capsule ; c, a Malpighian cap- 

 sule ; t, convoluted uriniferous 

 tube ; e 1 , small blood-vessels 

 from the capsule, which sub- 

 divide into capillaries p, sur- 

 rounding the tube, and finally 

 terminating in e, a small branch 

 of the renal vein. 



tubes) radiate towards the cortical portion, branching as they go. On reach 

 ing the cortical layer they are distributed irregularly, interlacing each other, 

 and finally terminate in little expansions called the Malpighian capsules. 

 Into each capsule a small branch of the renal artery enters, and immediately 

 breaks up into looped capillaries which nearly fill the cavity of the capsule. 

 The blood is then collected up by a small vein, which again breaks up into a 

 capillary network around the walls of the uriniferous tube, and is finally con- 

 veyed away through the medullary portion to the renal vein. Thus we have 

 another example (see page 155) of blood passing through two distinct capillary 

 systems before entering the great vein which takes it to the heart. 



The excretion of the kidney is filtered from the blood both through the 

 thin walls of the capsules and also through the walls of the tubules which are 

 surrounded by the second capillary network. It is probable that the watery 



