XXX.] THE BRAIN. 327 



d. The grey substance extending into GoU's 

 fasciculus of the posterior column, and into 

 the fasciculus euneatus. 



The appearances vary considerahly in sections taken 

 at different parts of the decussation. 



2. Take a brain of a rabbit' hardened in ammonium 

 bichromate and subsequently treated with al- 

 cohol. Cut out a piece from the middle of the 

 dorsal portion of one hemisphere, extending from 

 the surface to the lateral ventricle, and prepare 

 vertical sections (preferably with a freezing mi- 

 crotome). Stain then with dilute carmine or 

 picrocarmine, clear and mount in Canada balsam. 

 Observe 



a. The inner layer of horizontal nerve fibres 

 (meduUated) forming the white substance; 



I The fresh brain should be placed in anunoniam or potassium 

 bichromate 2 p.o., the fluid changed on the following day (when the 

 brain may be cut trausversely in four or five pieces) and again in a 

 week, then left about three weeks. It should then be cut up and the 

 pieces washed with water for a day and with dilute spirit (first 30 p.o. 

 then 50 p.c.) until all the excess of the chromium salt is taken out. 

 The pieces may be stained with strong Frey's carmine during a 

 week or more, washed well with water, soaked in gum and cut with a 

 freezing microtome. 



Instead of a rabbit's brain the brain of a cat or dog may be taken ; 

 a solution of the chromium salt may advantageously be injected into 

 the basilar artery, but even then it is not easy to preserve the natural 

 form of the cells of the 3rd and 4th layers of the cortex; the structure 

 of the cortex in the cat and dog differs also at different points more 

 than in the rabbit, the chief points of difference being the occurrence 

 of very large cells in groups or singly in the lower part of the third 

 layer, and the variation in number and extent of the angular cells, 

 which may spread into the third layer or may be inconspicuous, 



