380 CHAPTER XXX. 



hour wash in water ; stain with 0'25 per cent, aqueous solution of anilin blue- 

 black, followed by picro-carmine ; dehydrate and clear in pyridin ; mount 

 in balsam thinned with pyridin. See also 103. 



683. 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 

 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 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 

 with a weight attached to its lower end. The weight has the 

 double function of preventing any part of the preparation 

 from floating above the level of the hardening liquid (a thing 

 that easily happens where somewhat dense liquids, such as 

 Miiller's solution, are used), and of preventing the torsions of 

 the tissues that may otherwise be brought about by the elastic 

 fibres of the pia mater and arachnoid. 



