39.2 CENTEAL NERVOUS SYSTEM. 



Brains that are not fresh require for hardening longer time and stronger 

 alcohol. 



Instead of the iodine solution, it is' possible to use for the preliminary 

 hardening a mixture of equal volumes of chloroform and ether ; but this 

 mixture is not to be recommended, on account of its solvent action on proto- 

 plasm and on the processes of ganglion-cells. 



The methods of Betz are particularly adapted to the hardening of volu- 

 minous specimens, and of tissues that are in a state of post-mortem softening. 



740. Osmic Acid (ExNEE, Sitzb. Jc. Akad. Wiss. Wien, 1881, Ixxxiii, 

 3 Abth. ; BEVAN LEWIS, The Human Brain, p. 105). A small portion of 

 brain, not exceeding a cubic centimetre in size, is placed in ten times its 

 volume of 1 per cent, osmic acid. The solution should be replaced by fresh 

 after two days, a proceeding which may advantageously be repeated at the 

 end of the fourth day. In from five to ten days the piece is usually 

 hardened throughout, and may be washed with water, treated with alcohol, 

 and imbedded. The sections may be treated by a drop of caustic ammonia, 

 which clears up the general mass of the brain substance, leaving medullated 

 fibres black. B. Lewis says that this method exhibits a wealth of structure 

 which no other method displays. The sections may be mounted in soluble 

 glass. The chief value of this method is for tracing the course of medul- 

 lated fibres. 



741. BELLONCI (Arch. Hal. de Biol., vi, p. 405) employed this method in 

 his researches on the optic nerve of mammalia. He used an osmic acid 

 solution of 0'5 to 1 per cent., hardened for only fourteen to twenty hours, 

 made sections, and treated them for three or four hours with 80 per cent, 

 alcohol, and then with ammonia. 



742. Nervous Centres of Reptiles and Batrachia (MASON, Central 

 Nervous System of Certain Reptiles, &c. ; WHITMAN'S Methods, p. 196). 

 Iodised alcohol, six to twelve hours; 3 per cent, bichromate, with apiece of 

 camphor in the bottle, and to be changed once a fortnight until the harden- 

 ing is sufficient (six to ten weeks). 



BUKCZHAKDT (Das Centralnervensystem von Protopterus, Berlin, 1892 ; 

 Zeit.f. wiss. Mik., ix, 3, 1893, 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. 



Imbedding and Cutting. 



743. The Methods of Imbedding. The paraffin infiltration 

 method can only be used for the smaller objects of this class. 

 Human spinal cord (which is quite at the upper limit as re- 

 gards size) can be properly penetrated with paraffin by taking 

 the precaution of first cutting it up into slices of not more 

 than a few millimetres preferably not more than one in 

 thickness. The largest objects of this class, such as entire 



