CHAPTER XII. 



THE URINARY ORGANS. 



The urine is secreted by the kidney, whence it passes succes- 

 sively through the renal pelvis, ureter, bladder, and urethra into 

 the outer world. 



1. The kidney is made up of homologous parts or lobes, which 

 are readily distinguished in early life by the superficial furrows 

 marking their lines of junction. In later years these depressions 

 on the surface of the kidney disappear. Each of the lobes corre- 

 sponds to one of the papillae of the kidney and the pelvic calix that 

 embraces it. In some of the lower animals — e. g., the rabbit — the 

 kidney has but one papilla, so that the whole renal pelvis in those 

 animals corresponds to a single calix in man. 



The kidney is a compound tubular gland of peculiar construc- 

 tion, the tubules taking origin from little spherical bodies, called 

 Malpighian bodies, instead of from simple blind extremities, and, 

 after running a definite and somewhat complicated course, uniting 

 successively with several others to form the excretory ducts, called 

 the " collecting tubules," which open into the calices near the tips 

 of the papilla?. 



If a section of the organ be made through its convexity down to 

 the pelvis, the papilla? will be seen projecting into the calices of the 

 pelvis, and it will be noticed that each papilla forms the apex of a 

 pyramidal portion of tissue having a different tint and texture from 

 the rest of the kidney. These pyramids form the " medulla " of 

 the organ (Fig. 138). 



The bloodvessels supplying nearly all its substance enter the 



kidney near the bases of the pyramids, having approached the 



organ through the fat that lies around the calices. Within the 



kidney they break up into branches that run along the base of each 



pyramid in that portion of the organ which is called the "boundary 



zone." Between that zone and the convex surface of the kidney 



the tissue is known as the "cortex." 



The arrangement of the renal tubules, which make up the chief 

 n 161 



