208 THE BODY AT WORK 



crystals of uric acid. It must be confessed that, in whatever 

 way one attempts to account for the excretion of uric acid by 

 birds, the similarity of structure of their kidneys and those of 

 mammals is difficult to reconcile with the wide difference in 

 consistency and in chemical composition of the excrement. 



Reflecting upon all the evidence bearing upon the mechanism 

 of the mammalian kidney, the majority of physiologists come 

 to the following conclusions : The greatest outflow of water 

 occurs in the glomeruli. The water is accompanied by salts, 

 including a small quantity of urea. The contorted and spiral 

 portions of the tubule and the ascending limbs of Henle's loops 

 add to the urine the remainder of the urea, together with 

 various bodies still less readily diffusible. 



It may be that the chief function of the loops of Henle is to 

 oppose resistance to the passage of fluids, thus heading up the 

 secretion, and favouring the osmosis of water into it from the 

 blood of the glomerular capillaries. It is possible that the 

 calibre of the slender descending limbs is influenced by external 

 pressure, their partial occlusion being increased, and the 

 pressure in them raised, when the organ is very active and 

 its intermediate zone turgid with blood. 



Various drugs influence the secretion of the kidney. In 

 some cases their action seems to be mainly hydrostatic. They 

 change the rate of flow by altering blood-pressure. Digitalis in- 

 creases the force of the heart. The heart beating more strongly, 

 blood-pressure rises. Higher blood-pressure is accompanied by 

 a more copious secretion. This action of digitalis is far more 

 marked when the heart is out of order than when it is healthy. 

 In heart-disease the blood-pressure is unduly low, and the tissues 

 become water-logged in consequence. When the blood- 

 pressure is restored and a brisker capillary circulation estab- 

 lished, water and waste-products, which have accumulated in 

 lymph, pass, as they ought to do, into the veins. Carried into 

 the general circulation, they overflow from the kidney. 



It is a little difficult to realize the abundance of the body- 

 fluids. From one-quarter to one-third of the whole body- 

 weight is due to lymph, using this term in its most general sense. 

 The waste-products of tissues collect in the lymph. The 

 blood circulating through capillary vessels which traverse 

 lymph-spaces takes up water and waste-products. Its just 



