HARDENING BY REAGENTS. 385 



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. 



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 between 

 the operculum and the median lobe, and as far as possible 

 between the convolutions. Unless there are special reasons 

 to the contrary, the brain should be divided into two sym- 

 metrical 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 

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

 Mik., 1884, p. 388) finds that at a temperature of from 30 to 

 40 C. preparations may be sufficiently hardened in solution 

 of Muller 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 aban- 

 doned (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 tjiis reason 

 voluminous preparations that are to be hardened in the slow 



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