232 PRACTICAL HISTOLOGY. 



employed should include both cortical and medul- 

 lary parts; but at the same time should not be 

 thicker than from a quarter of an inch to half an 

 inch, otherwise the preservative fluid will not pene- 

 trate rapidly enough to the deeper parts. They are 

 to remain in the bichromate solution (a relatively 

 large quantity) for three weeks; are then placed in 

 weak spirit, and in twenty -four hours transferred to 

 strong spirit. In three or four days more the pieces 

 are firm enough to cut. They will perhaps be large 

 enough to hold in the hand, and thus the necessity 

 of imbedding will be avoided; sections as thin as 

 possible are to be made in a plane vertical to the 

 surface of the organ, and large enough to include 

 both cortical substance and Malpighian pyramid. 

 One such section is first -transferred from the spirit 

 to water, and is then simply mounted in glycerine; 

 another is stained with logwood, and after treatment 

 by the customary process is mounted in dammar 

 varnish. These sections will show: in the cortical 

 substance the Malpighian corpuscles, the convoluted 

 tubules variously cut, and the prolongations of the 

 straight tubules of the medullary substance and of 

 the tubules of Henle ; in the pyramidal part the two 

 last-named tubules, and the collecting and excretory 

 tubules, seen longitudinally, with a large number of 

 bloodvessels running parallel to and between them. 



Preparation 6. A transverse section also should 

 be obtained of the medullary substance. With this 

 object the end of a pyramid is cut off with a razor, 

 and, as it is too small to hold in the hand, it is im- 

 bedded in wax-mass in such a position that the 

 tubules will be cut exactly across. Sections from 

 this are mounted like the others in glycerine and in 

 dammar varnish. 



The kidney of the dog or of almost any other ani- 

 mal may be used for the above preparations, and has 

 the advantage over the human organ, that it is ob- 

 tainable in a fresher condition. But since the epi- 

 thelium of the tubules, at all events of the convoluted 



