SECTION II 



THE SECRETION OF URINE 



WITH the single exception of hippuric acid, all the constituents of the urine 

 are formed in parts of the body other than the kidneys. Extirpation of 

 both kidneys leads to an accumulation of these specific urinary con- 

 stituents in the blood and tissues. The work of the kidney is therefore 

 confined to an excretion of preformed constituents. Considered from a 

 broad standpoint, the function of this organ is the preservation of the normal 

 composition of the circulating blood. 

 Whenever the latter contains an abnor- 

 mal constituent or any of its normal 

 constituents are present in abnormal 

 quantities, the kidney excretes the sub- 

 stance in question until the composition 

 of the blood is restored. We have to 

 determine the conditions which influence 

 the quantity and quality of the urine 

 secreted by the kidneys, and to ascribe 

 to each element in these organs its proper 

 share in the total work of the kidney. 



In no other organ of the body are our views 

 as to function so intimately dependent on our 

 knowledge of structure as in the kidney. This 

 organ is a branched tubular gland consisting 

 in man of ten to fifteen nearly equal divisions, 

 known as the Malpighian pyramids. In certain 

 animals, such as the rabbit and rat, only one 

 pyramid is present. It is divided into an outer 

 portion or cortex, an inner portion, the medulla, 

 and between these the ' boundary layer,' con- 

 taining the larger branches of the renal blood 

 vessels (Fig. 541). From the outer boundary 

 of the Malpighian pyramids of the medulla, a 



number of processes, the medullary rays, pass out into the cortex towards the surface 

 of the kidney. All parts of the kidney are made up of branched tubules embedded in 

 scanty connective tissue and richly supplied with blood vessels. Each tubule begins 

 by a blind dilated extremity in the cortex, known as Bowman's capsule, which surrounds 

 a bunch of capillary blood vessels, the glomerulus, the two together forming the Mal- 

 pighian body. From Bowman's capsule a short neck leads into a proximal convo- 

 luted tubule, and this into a y- shaped portion which passes down in a medullary ray 



1181 



FIG. 541. Section of human kidney. 

 (CADI AT.) 



a, cortex; &, medulla or Malpighian 

 pyramids; c, papilla; d, ureter; 

 e, e, boundary zone. 



