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 constituents 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 ol this organ is the 



preservation of the normal com- 

 position of the circulating blood. 

 Whenever the latter contains an 

 abnormal constituent or any of its 

 normal constituents are present in 

 abnormal quantities, the kidney 

 excretes the substance 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 inti- 

 mately dependent on our know- 

 ledge 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,' containing the 

 larger branches of the renal blood-vessels (Fig. 526). 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 



1264 



Section of human kidney 



(CADIAT.) 

 a, cortex ; Z>, medulla or Malpighian 

 pyramids; c, papilla; d, ureter; 

 e, e, boundary zone. 



