NERVOUS SYSTEM— GENERAL METHODS. o75 



739. 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 ont on 

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

 they may have the hardening liquid poured over them. Or, 

 still better, the preparations may bo S7ts2>rn<led in the liquid, 

 see § 34. 



Another plan, which is good, is to add to the hai'dening 

 liquid enough glycerin or salt to make the pi-eparations 

 juiit float. 



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 orgnns without 

 dividing them into portions, they should at lenst be incised 

 as deeply as possible in the less important I'egions. 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. ^^'ith material intended for the 

 Golgi impregnation it is well not to remove them at all. 



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, in order to 

 counteract the torsions of the tissues that may otherwise occur. 



1''he certihrum should have plugs of cotton-wool put into 

 the fissure of Sylvius, and as far as possible between the 

 convolutions. Unless there are special reasons to the con- 

 trary, the brain should be divided into two symmetrical 

 halves by a sagittal cut passing through the median plane 

 of the corpus callosum. Betz recommends that after a few 

 hours in the hardening liquid the pia mater should be re- 

 moved wherever it is accessible, and the choroid plexuses also. 



The cerehellnm should be treated after the same manner. 

 The hardening action of most solutions is greatly onchanced 

 by heat. Thus Weigeet (Centralb. med. 11'iss., 1882, 

 p. 819; Zeit. wiss. Mile, 1884, p. 388) finds that at a 



