196 THE BODY AT WORK 



vessels in which the products of digestion circulate, needs an 

 organ which provides for the overflow from the body-fluids of 

 all substances which are injurious or effete. 



The kidney is an aggregation of long urinary tubules. The 

 head of each tubule is dilated into a globular capsule, into 

 which a tuft of bloodvessels depends. This is the sink into 

 which the waste- water of the blood drips. The long urinary 

 tubules are lined with cells well qualified by form and con- 

 stitution to search the blood in the capillaries which border 

 them, for substances which, not being easily diffusible, 

 have to be forcibly dragged from it and added to the water 

 trickling down the pipe which connects the rain-water head 

 with the sewer. The hydrostatic conditions of this apparatus 

 the provision for greater or less flow of blood through the tufts 

 (glomeruli) which hang in the capsules, and for longer or shorter 

 exposure of the blood to the purifying activity of the epithelium 

 of the renal tubules will be described after a very brief account 

 has been given of the structure of the organ. 



The outer border of the kidney is convex, its inner border 

 concave. The concavity is termed the "hilus." The central 

 depression of the hilus is embraced by the expanded end of 

 the ureter the tube which carries the secretion of the kidney 

 to the bladder. The renal artery and the renal nerves enter, 

 and the renal vein leaves, the kidney at the hilus. 



If a kidney be split longitudinally, it will be noticed that its 

 outer part, the cortex, is darker in colour than its inner part, 

 the medulla (Fig. 9). The glomeruli already referred to occur 

 in the cortex. The medulla is occupied by radiating tubules, 

 collected into groups. Those of each group converge towards 

 a common duct. From twelve to eighteen ducts open into the 

 expanded end of the ureter, each at the apex of a pyramid. If 

 the section of the kidney be examined with a lens, it will be 

 seen that narrow rays from the medulla extend into the 

 cortex. The cortex is therefore made up of interdigitating 

 pyramids of dark substance, consisting of glomeruli and the 

 contorted tubules, about to be described, and of lighter sub- 

 stance, consisting of straight tubules continuous with those of 

 the medulla. 



The urinary tubules are the separate pieces of apparatus 

 of which the kidney consists. The problems connected with 



