NEUROLOGICAL METHODS. 381 



The cerebrum should be very delicately laid out on a layer 

 of cotton wool, or, if possible, hung up in it. Plugs of the 

 wool should be put into the fissure of Sylvius, and as far as 

 possible between the convolutions. Unless there are special 

 reasons to the contrary, 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 removed wherever it is accessible, and the choroid plexuses 

 also. I have found this by no means easy, and think it is 

 an operation that can only be recommended for experienced 

 hands. 



The cerebellum should be treated after the same manner. 



The temperature at which the preparations are kept in the 

 hardening solution is an important point. The hardening 

 action of most solutions is greatly enhanced by heat. Thus 

 WEIGERT (Centralb. f. d. med. Wiss., 1882, p. 819; Zeit. f. 

 wiss. Mik.j 1884, p. 388) finds that at a temperature of from 

 30 to 40 C. preparations may be sufficiently hardened in 

 solution of Miiller in eight or ten days, and in solution of 

 Erlicki in four days ; whilst at the normal temperature two or 

 three times as long would be required. 



But it is not certain that this rapid hardening always gives 

 the best definite results. SAHLI, who has made a detailed 

 study of the hardening action of chrome salts, is of opinion 

 that it does not, and thinks it ought for this reason to be 

 abandoned (see Zeit. f. wiss. Mik., 1885, p. 3). 



On the other hand, the slowness of the action of chromic 

 salts at the normal temperature is such that decomposition 

 may easily be set up in the tissues before the hardening and 

 preserving fluid has had time to do its work. For this reason 

 voluminous preparations that are to be hardened in the slow 

 way should be put away in a very cool place best of all in 

 an ice-safe. 



See also the paper of PFISTEB on the preliminary hardening of Central 

 Nervous System in situ, in Neurol. Centralb., xvii, 1898, p. 643 (Zeit. 

 f. wiss. Mik., xv, 4, 1899, p. 494). 



684. The Reagents to be employed. The hardening agents 

 most used are the chromic salts. Chromic acid was much 

 used at one time, but most workers now agree that its action,. 



