SECTION CUTTING. 125 



corrosive sublimate, but the foregoing answer all practical 

 purposes. 



When there are bone or lime salts in the tissues, chromic acid 

 is the most useful reagent. It serves at the same time to 

 harden the soft portions, and to decalcify and thus soften the 

 hard. The object must be placed in a large quantity of the 

 fluid of a greater strength (one to two per cent.) and the 

 acid should be frequently changed until all lime salts have 

 disappeared. Of course, with this increase in strength of acid 

 and the length of time of immersion, one runs a risk of the 

 other portion becoming brittle, but this cannot well be 

 avoided. In some cases it is necessary to use dilute hydro- 

 chloric acid in place of the chromic. This should rarely be 

 used of greater strength than one per cent. After the sub- 

 stance is thoroughly decalcified it is transferred to alcohol. 



Frequently specimens contain such a large amount of pig- 

 ment matter as to render the thinnest section opaque and to 

 utterly obscure all cell limits. In such cases it is necessary 

 to immerse the tissues in 25 per cent, nitric acid, and to watch 

 closely until the color disappears. This usually takes some 

 hours, and the sections cut from such material are not very 

 satisfactory but are the best that can be obtained. 



The process of embedding comes next in order. The sub- 

 stances used are many, the most common being pure par- 

 affine, a mixture of paraffine and oil or tallow, wax and tallow, 

 transparent soap, gum arabic, and glycerine jelly. With most 

 substances paraffine, without the admixture of anything, gives 

 the best results, though many advise the addition of a fourth 

 to a half of paraffine oil, or lard, or tallow, to render it softer. 



