384 CENTRAL NERVOUS SYSTEM. 



mucilaginous or gelatinous freezing mass ( 316, et seq.), in order to avoid 

 the formation of ice crystals in the tissues. But in the case of fresh tissue, 

 the ether freezing method is to be preferred. This method allows of rapidly 

 producing any desired degrees of hardness, and maintaining them or allow- 

 ing them to diminish as occasion may require, and is perhaps the only 

 method by which satisfactory sections of unhardened nerve-tissue can be 

 obtained. 



The sections should be floated on to water, treated for a minute on the 

 slide with 0*25 per cent, osmic acid solution, and stained or otherwise treated 

 as desired. 



.For a detailed description of these manipulations see BEVAN LEWIS'S The 

 Human Brain. 



730. Generalities on Hardening by Reagents. If large pieces 

 of nerve-tissue are to be hardened, it is necessary to take 

 special precautions in order to prevent them from becoming 

 deformed by their own weight during the process. Spinal 

 cord or small specimens of any region of the encephalon may 

 be cut into slices of a few millimetres thickness, laid out on 

 cotton wool, and brought on the wool into a vessel in which 

 they may have the hardening liquid poured over them. The 

 wool performs two functions : it forms an elastic cushion on 

 which the preparations may lie without being distorted by 

 their own weight ; and it allows the reagent to penetrate by 

 the lower surfaces of the preparations as well as by their ex- 

 posed surfaces. A further precaution, which is useful, is to 

 hang up the preparations, lying on or in the cotton wool, in a 

 glass cylinder or other tall vessel ; by hanging them near the 

 top of the liquid the processes of diffusion and the penetra- 

 tion of the reagent are greatly facilitated. 



If the preparations are placed on the bottom of the vessel, 

 they should never be placed one on another. 



If it be desired to harden voluminous organs without divid- 

 ing them into portions, they should at least be incised as deeply 

 as possible in the less important regions. It is perhaps better 

 in general not to remove the membranes at first (except the 

 dura mater), as they serve to give support to the tissues. The 

 pia mater and arachnoid may be removed partially or entirely 

 later on, when the hardening has already made some pro- 

 gress. 



The spinal cord, the medulla oblongata, and the pons Varolii 

 may be hardened in toto. The dura mater should be removed 

 at once, and the preparation hung up in a cylinder-glass, 



