326 CENTRAL NERVOUS SYSTEM. 



and the median (central) lobe, in the direction of the de- 

 scending cornu, and between the convolutions) . After twenty- 

 four to seventy-two hours the brain is brought into fresh 

 solution of iodine in 70 per cent, alcohol, where it remains 

 until the hemispheres are hard enough to be supported on two 

 fingers without bending. (This will not be before ten to four- 

 teen days). It is then put into 4 per cent, solution of 

 bichromate and left to acquire its definitive hardness. If an 

 excessive brown deposit make its appearance, and the brain 

 be found notwithstanding to be not hard enough for cutting, 

 it must be rinsed with water and the bichromate solution 

 changed. When ripe for cutting the brain ought to show 

 an almost equal intensity of yellow-brown stain over the 

 whole surface of a cut made through the total thickness of a 

 hemisphere. 



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 chloro- 

 form and ether ; but this mixture is not to be recommendedi 

 on account of its solvent action on protoplasm and on the 

 processes of ganglion-cells. 



The methods of Betz are particularly adapted to the harden- 

 ing of voluminous specimens, and of tissues that are in a state 

 of post-mortem softening. 



669. Osmic Acid (ExNEE, Sitzb. k. 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 im- 

 bedded. 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 medullated fibres. 



BELLONCI (Arch. Ital. de BioL, vi, p. 4Q5) has been recently employing 

 this method in his researches on the optic nerve of mammalia. He employs 

 an osmic acid solution of 0'5 to 1 per cent., hardens for only fourteen to 

 twenty hours, makes sections, and treats them for three or four hours with 

 80 per cent, alcohol, and then with ammonia. 



670. GOLGI'S Method. See below, 673, subjmem. 



