THE URINE 



315 



The vascular arrangements of the kidney are intimately 

 connected with the function of the organ. The renal artery is 

 short, it comes off close to the posterior aorta, and the pressure 

 within it is practically the pressure in that vessel ; the pressure 

 in the renal vein, on the other hand, is low, nearly as low as that 

 in the posterior vena cava. It will be observed that the same 

 amount of blood-pressure as is required to fill the vessels of the 

 lumbar region and hind-limbs is expended on driving the blood 

 through the kidneys. At every increase in the amount of blood 

 in the kidney the organ swells, at every decrease it contracts. 

 These movements on the part of the kidney have been carefully 

 studied by means of Roy's oncometer. An oncometer (Fig. 92) is 

 a metallic capsule in which the living kidney is enclosed, and so 

 arranged that the 

 expansion and col- 

 lapse of the organ 

 can readily be re- 

 corded. A tracing 

 given by the use 

 of this instrument 

 shows that the 

 volume of the kid- 

 ney is affected by 

 every beat of the 

 heart, and even by 

 the respiratory un- 

 dulations in the 

 blood-pressure. 



Structure of the 

 Kidney. — The kid- 

 ney consists of a 

 central part, the 

 medulla,surrounded 



by an external part, the cortex ; the boundary of the two is 

 easily visible in a sliced kidney. The branches of the renal 

 artery break up at the boundary of the cortical and medullary 

 portions. The cortex of the kidney is the essential secreting 

 region, and it is here that the Malpighian tufts or capsules are 

 found. These consist of small balls of capillaries, the glomeruli, 

 derived from the renal artery ; the artery entering the Mal- 

 pighian tuft is larger than the vein leaving it, the result is 

 that a high blood-pressure is maintained in the glomerulus. 

 The vessel which supplies these tufts also sends* branches to 

 form a plexus around the uriniferous tubules ; these branches 

 do not enter the Malpighian body. The whole glomerulus is 

 contained in a capsule in which it is suspended by its afferent 



Fig. 92. — Diagram of Oncometer. 



B, Metal box in two halves, opening on the hinge H ; 

 M, thin membrane ; A, space filled with oil ; O, 

 organ enclosed in oncometer ; V, vessels of organ ; 

 t, tube for filling instrument with oil ; T, tube 

 connected with D, which opens into cylinder C ; 

 P, piston attached by E to writing lever. 



