208 MANUAL OF HISTOLOGY. 



toward the centre of the lumen of the tubule, and the distance 

 between any two adjoining rods at the periphery is necessarily 

 greater than at the centre. For the same reason, also, the rods 

 are more distinctly defined in the former situation ; the micro- 

 meter screw will have to be used in tracing them inward. 



Transverse sections of the cortical substance may be made 

 by freezing small pieces which have been immersed in a solu- 

 tion of the neutral chromate of ammonia. Such sections should 

 be examined in glycerine, or, better, in a saturated solution of 

 the chloride of potassium in glycerine. 1 The radial direction 

 of the rods is beautifully seen in such specimens (Fig. 95, C), 



FIG. 95. Kidney of dog. Transverse section through the medullary portion, about midway between 

 the apex and boundary layer. Neutral chromate of ammonia preparation : A, blood-vessel ; B, looped 

 tubule, descending portion ; C, looped tubule, ascending portion ; D, collecting tubule ; E, connective 

 tissue, x 300. 



and the individual cells are more clearly defined. Another 

 method of exhibiting these rod epithelia is to inject the artery 

 or vein of a fresh, bloodless kidney with a cold saturated solu- 

 tion of the chloride of potassium, then, after placing the whole 

 organ in alcohol, divide it in small pieces under that fluid. 

 After a day or two sections may be made ; they then should be 

 immersed for a short time in absolute alcohol and clarified by 

 oil of turpentine. Such specimens show the epithelium to 

 perfection and may be preserved for a considerable time. Per- 

 manent specimens can be made by substituting resinous turpen- 



1 The glycerine should be heated in a porcelain evaporating-dish, the chloride of 

 potassium added, and the whole mixture stirred for several minutes with a glass rod. 

 The glycerine is ready for use after cooling. 



