CHAPTER XXXI. 399 



The hardening and section cutting by the freezing method of very 

 large pieces require special apparatus and special methods, for which 

 see NAGEOTTE, C. R. Soc. BioL, Ixvii, 1909, p. 542. 



808. Hardening by Reagents. If large pieces of nervous tissue are 

 to be hardened, it is necessary to take special precautions in order 

 to prevent them from being deformed by their weight during the 

 process. The spinal cord or small portions of any region of the 

 encephalon may be cut into thin slices, laid out on cotton wool in a 

 vessel into which the hardening fluid is poured. The specimens may 

 also be suspended in the liquid ( 34). Another good plan consists 

 in adding to the hardening fluid just enough glycerine or sodium 

 chloride to make tissues float. 



If several pieces are placed in the same vessel, they should never 

 be put on top of each other. Voluminous organs to be hardened 

 in toto should be at least incised as deeply as possible in the less 

 important regions. With the exception of the dura mater, the 

 membranes are not generally removed at first, as they serve to 

 protect the tissues. They can be removed partially or entirely later 

 on when the hardening has made some progress. In the case of 

 material intended for Golgi's methods it is best not to remove them 

 at all. 



The spinal cord, medulla oblongaia and pom Varolii may be 

 hardened in toto, and the preparation hung up in a cylindrical 

 vessel with a weight attached to its lower end to prevent it from 

 becoming distorted. 



The cerebrum should have light plugs of cotton wool in the 

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

 If it is desired not to open the lateral ventricles, the hardening fluid 

 may be injected into them. Unless there are special reasons to 

 the contrary, the brain should be divided into two portions by a 

 middle frontal section, or better into two symmetrical halves by 

 a sagittal cut passing through the median plane of the corpus 

 callosum. 



The cerebellum should be treated in the same manner. 



The action of most hardening fluids is greatly enhanced by heat. 

 But in the judgment of most histologists this rapid hardening is not, 

 as a rule, attended by good results, and one should have recourse to 

 it only for particular reasons and special purposes after a tentative 

 experiment, whenever possible, at establishing the degree of tem- 

 perature at which the desired results may be obtained without 

 otherwise injuring the delicate structure of the nervous tissues. 



