NKKVOUS SYSTEM CIKNURAL MKTHODS. 375 



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 out 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 be suspended in the liquid, 

 see 34. 



Another plan, which is good, ir to add to the hardening 

 liquid enough glycerin or salt to make the preparations 

 just 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 vohuninous 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. With 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. 



The cerebrum 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. SETZ 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 cerebellum should be treated after the same manner. 

 The hardening action of most solutions is greatly enchanced 

 by heat. Thus WEIGERT (Centralb. med. Wi*s., 1882, 

 p. 819; Zeit. u-iss. Mik. t 1884, p. 388) finds that at a 



