462 NERVOUS SYSTEM— GENERAL METHODS 



cent, bichromate, changed once a fortnight until the hardening is 

 sufficient (six to ten weeks). 



BuRCKHARDT (Dtts Centralnervensysteni von Protopterus, Berlin, 1892 ; 

 Ztschr. iviss. Mikr., ix, 1892, p. 347) recommends a liquid composed of 

 300 parts of 1 per cent, chromic acid, 10 parts of 2 per cent, osmic 

 acid, and 10 parts of concentrated nitric acid, in which brains of 

 Protopterus are hardened in twenty-four to forty-eight hours. 



Fish (Journ. of MorphoL, x, 1895, p. 234) employed for Desmognathus 

 a mixture of 1000 c.c. of 50 per cent, alcohol, 5 c.c. of glacial acetic 

 acid, 5 grm. of corrosive sublimate, and 1 grm. of picric acid, fixing 

 for twelve to twenty-four hours, and passing through the usual alcohols. 



Strong {Journ. comp. NetiroL, xiii, 1903, p. 296) fixes (and decalcifies 

 at the same time) the heads of young Acanthios in a mixture of 9 parts 

 of 5 per cent, iron alum and 1 part of formol, for about two weeks, 

 makes paraffin sections, stains with haematoxylin, and differentiates in 

 1 or 2 per cent, iron alum. 



Johnston {Gegenbaur''s MorphoL Jahrb., xxxiv, 1905, p. 150) recom- 

 mends for Petromyzon fixing in Zenker's fluid and imbedding in paraffin. 

 The sections are stuck to slides and stained for several hours with alum 

 carmine. After rinsing in water they are placed in a mixture arrived at 

 by trial of saturated watery solution of nigrosin and saturated watery 

 solution of picric acid plus 1 per cent, acid fuchsin. 



IMBEDDING AND SECTION CUTTING 



988. Imbedding is by no means always necessary, and is objected 

 to in some cases. Indeed, sections can be made from any part 

 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 arable or of celloidin. 

 As soon as it begins to stick to the support the whole is put into 

 70 or 80 per cent, alcohol to harden the gum or the celloidin, 

 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 often cut by freezing methods, these being 

 very largely used since the introduction of COo microtomes, by 

 means of which many and relatively very thin sections can be 

 rapidly obtained with great economy of time and without im- 

 bedding media. Imbedding in gelatin may be resorted to in 

 special cases. The blocks must then be cut by means of the 

 freezing microtome and the sections mounted in glycerin jelly, 

 Apathy's syrup or some similar medium (§§ 463 et seq.). 



Imbedding in paraffin is not advised for the nervous system 

 in general, particularly after fixation in alcohol, and bichro- 



