318 CENTRAL NERVOUS SYSTEM. 



laginous or gelatinous freezing mass ( 308, 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 allowing 

 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 O25 per cent, osmic acid solution, and stained or otherwise treated 

 as desired. 



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

 Human Brain. 



659. 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 becom- 

 ing 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 pene- 

 tration 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 dividing 

 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 progress. 



The spinal cord, the medulla oblong at a, 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, with a weight attached to its lower end. The weight 

 ha> tin- double function of preventing any part of the prepara- 



