10 ON SECTION CUTTING. 



inasmuch as it does not colour the specimens, acetic acid may 

 conveniently be mentioned here as it is used for the same pur- 

 pose as the true stains, i.e., for the sake of rendering certain 

 parts of the cells especially distinct. Acetic acid, of which a 

 1 per cent, solution is employed, causes the protoplasm of cells 

 to swell uj) and become transparent, and brings the nuclei into 

 special prominence. It is used with fresh specimens. 



VII.-ON SECTION CUTTING. 



Many tissues and organs can only be studied satisfactorily by 

 cutting them into thin sections, and this method of investiga- 

 tion is of such importance as to require special notice. There 

 are three chief stages : Hardening, Imbedding, and Cuttings 

 which will be noticed in succession. 



A. Hardening. 



Before the object can be cut into sections it is necessary to 

 harden it ; this may be effected by freezing, but the more usual 

 plan is by means of reagents. The general action of these 

 hardening reagents is to coagulate the protoplasm of the tissues ; 

 and the objects to be attained are to effect this coagulation 

 quickly, before the tissues can undergo any alteration ; and 

 thoroughly, i.e., throughout the w^hole thickness of the object to 

 be hardened. To ensure the latter result it is always advisable 

 to use small pieces of the substance to be cut. 



The hardening reagents in most common use are as follows. 



1. Alcohol. Specimens may be placed at once in 70 per 

 cent, alcohol ; and thence transferred after a couple of days to^ 

 90 per cent., in which they may be left till required. 



2. Osmic Acid. For this purpose a 1 per cent, solution in 

 water is used : it acts almost instantaneously, and so allows no 

 change to occur in the tissues ; it has also the merit of staining 

 the tissues as well as hardening them. It can, however, only 

 be employed when the specimens are very small, as it hardens 

 the surface layers so rapidly that it is unable to penetrate 

 beyond a very slight depth. A few minutes immersion is 

 usually sufficient. 



3. Corrosive Sublimate, A saturated solution in water is 

 employed, in which the object is placed for half an hour or more. 

 After removal it is thoroughly washed with water or weak alcohol, 

 and then transferred to 70 per cent, alcohol before staining. 



