406 NERVOUS SYSTEMGENERAL METHODS. 



of the central nervous system without it, if the tissues are well 

 hardened. Material hardened in alcohol, or in chromic solutions, 

 or treated according to Golgi's methods may be glued on to a piece 

 of wood or hard cork (or still better to a glass cube) by means of a 

 rather thick solution of gum arabic. As soon as it begins to stick 

 to the support the whole is put into 70 to 80 per cent, alcohol to 

 harden the gum, and then fixed in the object-holder of the microtome 

 and cut. Or one can simply make a clean cut at the bottom of the 

 specimen, dry it with blotting paper and stick it on the support with 

 sealing wax or paraffin of high melting point. For section cutting 

 the knife should be wetted with alcohol or water ; if the latter is 

 used some soap may be added to it to prevent it from running into 

 drops on the knife. 



Formalin material is preferably cut by the freezing method, this 

 being very largely used since the introduction of C0 2 microtomes, 

 by means of which many and relatively very thin sections can be 

 rapidly obtained with great economy of time and imbedding reagents. 



Imbedding in paraffin is not advised for the nervous system 

 in general, particularly after fixation in alcohol, and bichromate 

 solutions. One should have recourse to it only for special 

 cytological methods, taking care not to use paraffin of too high 

 a melting point. 



Imbedding in celloidin is very largely used, and to great advantage, 

 for many purposes. 



If, notwithstanding every precaution, the celloidin has not 

 thoroughly penetrated the tissues, good sections may still be obtained 

 by DUVAL'S method of collodionising the sections. The cut surface 

 of the block is dried by blowing on it, and is covered with a thin 

 layer of collodion laid on it with a brush. As soon as this layer has 

 somewhat dried, which happens very rapidly, a section is cut, and 

 the cut surface collodionised as before, and so on for each section. 

 This process gives very good results, and may be advantageously 

 employed even with material that has been successfully imbedded, 

 as it gives greater consistency to brittle or otherwise delicate 

 tissues. 



The above applies to section cutting of small, medium-sized and 

 even relatively large pieces. Also unusually large pieces, entire 

 human hemispheres, and brains of high vertebrates can be cut into 

 thin, and, if necessary, serial sections both without, and after im- 

 bedding either in celloidin or paraffin or by mixed methods. The 

 processes used for the purpose do not differ essentially from those 

 above-mentioned and fully described in Chapters VII., VIII. and 



