STRUCT V BE OF THE KIDNEY 217 



the cortex as the ascending limb. This leads into the 

 second convoluted tubule, beyond which it unites with 

 neighbouring tubules to form one of the junctional tubules 

 which traverse the medulla radially and open at the apex 

 of a pyramidal projection into the pelvis of the kidney. 



The cells hning the tubule differ in different parts. In 

 the first and second convoluted tubules and in the upper 

 half of the descending hmb the cells are high columnar, 

 with well-marked striations formed of rows of granules 

 in the outer half and at the inner free border, which is 

 cihated. Though the tube is wide the lumen is small. 

 In the rest of the tubule, the lumen is wider ^nd the cells 

 hning it are flattened. 



Blood Suppkj. — ^In the surface of separation between 

 cortex and medulla, the renal vessels form an arcade. 

 The artery gives off branches which traverse the cortex 

 radially. From these branches arise lateral twigs which 

 lead into the glomeruh'. The venules draining these break 

 up to form a network surrounding the tubules. From this 

 network the blood is conveyed to the renal veins. 



The blood supplying the kidney, therefore, hke that 

 supplying the viscera, passes through a double system of 

 capillaries. It is now estabhshed that the tubules receive 

 no direct arterial supply. 



Nerve Supply. — The renal plexus situated at the hilum 

 of the kidney receives fibres (1) from the sympathetic, 

 emerging from the lower thoracic segments of the cord, and 

 (2) from the vagus. 



The action of the sympathetic is to cause diminution of 

 secretion by vaso-constriction. Though this nerve is said 

 to contain vaso-dilator fibres, its constrictor influence is 

 paramount. 



The function of the vagus is unknown. 



The kidney appears to be supphed with no secretory 

 fibres. Although it is hberally supplied with nerves, these 

 are not essential to its activity. The kidney when excised 

 and immediately replaced soon resumes its functions. 



