156 PRACTICAL HISTOLOGY. 



be referred to under the head of staining agents ( 327, 



330, 33I-) 



282. Freezing is a method of very great value. With 

 its aid, lung, kidney, spleen, lymphatic gland, vitreous 

 humour, brain, etc., may all be removed from the living 

 animal, frozen, and sliced, within fifteen minutes. 



Freezing is suitable for hardening the brain and spinal 

 cord, only when slices of the fresh tissue are desired for the 

 isolation of nerve cells, etc. These organs are best hardened 

 in chromic acid, potassium bichromate, etc., not because 

 they can be more readily cut when hardened by these 

 substances, but for the reason that, when the tissue has 

 been previously hardened by these agents, the slices do not 

 go to pieces when they are manipulated, as they are apt to 

 do when the unhardened frozen tissue thaws after freezing. 

 For the same reason, it is advantageous, in the case of the 

 kidney, lung, liver, spleen, and such like, to have the pro- 

 cess of hardening carried to some extent by Miiller's fluid, 

 potassium bichromate, chromic or picric acid, before freezing 

 them for the purpose of section. The special cases where 

 freezing is of service have been indicated in the preceding 

 demonstrations. 



Indeed, speaking generally, freezing is of most service 

 i. In enabling us to quickly harden and cut fresh tissues 

 when the permanent preservation of these is not a primary 

 consideration. 2. In completing, with a view to section, the 

 hardening of tissues already partially indurated by other 

 means ; so that a. The process of hardening is shortened ; 

 b. The tendency to shrivelling of the tissue by the harden- 

 ing fluid is lessened, because of its shorter exposure to its 

 influence ; c. The sections, when they thaw, are still suffi- 

 ciently indurated by their treatment previous to freezing, to 

 permit of their successful manipulation and preservation. 

 Freezing may be employed after any of the hardening 

 agents described in the preceding paragraphs, but in the 

 case of alcohol, it of course must be completely removed 

 by soaking the tissue in water ere the freezing can be 

 accomplished. 



Freezing has hitherto been had but little recourse to, 



