XXV] KIDNEY 195 



6. Observe under a high power 



a. In the medulla: 



The epithelium of the collecting tubes; in the 



smaller tubes this is composed of short columnar or 

 cubical cells with spherical or ovoid nuclei; in the 

 larger tubes (outflow tubes) it is composed bf longer 

 columnar cells with ovoid nuclei and more transparent 

 cell-substance ; the lumina, distinct throughout, become 

 larger as the tubes increase in size. 



13. In the intermediate layer : 



The continuation of the collecting tubes outwards 

 in the medullary rays. 



The loops of Henle, seen as small straight tubes, 

 chiefly in the medullary rays (the bend of the loop is 

 not easily made out in this specimen). They run also 

 a variable distance into the medulla. 



The ascending limbs of the loops ; these will 

 probably be deeply stained, the cells have a rather 

 marked striated outer portion. 



The descending limbs of the loops; they are 

 about half the diameter of the ascending limbs and 

 have shallow or flat cells. 



In some animals the cells of the ascending limb are placed 

 obliquely, and the nuclei (in sections) are oval ; the cells of the 

 descending limb may be flat and transparent with nuclei pro- 

 jecting into the lumen, except for the thick basement membrane, 

 they then resemble somewhat blood capillaries. The change in 

 character of the epithelium may take place either in the ascending 

 or in the descending limb of the loop. The lumina may be 

 barely perceptible or about half the diameter of the tube, accord- 

 ing to the state of distension. 



The numerous blood vessels between the medullary 



132 



