400 NERVOUS SYSTEMGENERAL METHODS. 



On the other hand the hardening action at room temperature of 

 certain reagents, such as solutions of chromic salts, proceeds so 

 slowly that decomposition may set in before the fluid has had time 

 to act effectively. For this reason voluminous preparations which 

 are to be hardened in toto in solutions of chromic salts, and were 

 not injected as described in 806, should be put away in a very 

 cool place or in an ice-chest. A human cerebral hemisphere may 

 require eight or nine months for hardening in this way. 



The volume of the fluid should always be very large in proportion 

 to that of the pieces of tissue and to their number. It should be 

 taken in solutions as weak as is consistent with the proper preserva- 

 tion of the tissues. It should be frequently changed and its strength 

 gradually increased. 



MARIE'S method of fixing and hardening in situ is highly recom- 

 mended; for its indications and contra-indications, see SAINTON 

 and KATTWINKEL (Deutsche Arch. klin. Med., Ix, 1898, p. 548) and 

 PFISTER (Neurol Centrbl, xvii, 1898, p. 643). 



809. The Reagents to be Employed. As in the case of the fixation 

 by injection one should bear in mind that the preservation of tissues 

 for neuro-histological investigations greatly depends upon the 

 purpose in view. Fixing and hardening fluids which are excellent 

 for cytological investigations are very often unsuitable for anatomical 

 methods. (See 805.) On the other hand, material collected and 

 prepared for cyto-architectonic or fibro-architectonic studies can 

 hardly be used to elucidate questions regarding the intimate structure 

 of nerve-cells or nerve-fibres. Alcohol, formalin * and chromic salts 

 are most frequently used because they are generally ready at hand, 

 and because they are useful for carrying out afterwards either a 

 great number of methods, or certain methods, under constant 

 conditions of hardening and staining. 



810. Alcohol. It is generally employed in the strength of 94 to 96, 

 per cent., penetrates well and hardens quickly ; but as it rapidly 



* Wrong as it is, I find it expedient to use the term " formalin " or 

 " formol " in the generally accepted sense, viz., as if it were a chemical 

 reagent, while it is only a commercial denomination which ought not 

 to have been introduced in the histological terminology. See 108. 

 As is well known, the commercial formalin is only a 40 per cent, solution 

 of formaldehyde ; but when in this and the following chapters on the 

 nervous system a 5, 10 or 20 per cent, solution of formalin is mentioned, 

 it is intended to mean 5, 10 or 20 parts of commercial formol, and 95, 

 90 or 80 parts of water, respectively, while, e.g., a 20 per cent, solution 

 of formaldehyde is the commercial formalin diluted with half its volume 

 of water. C. d. F. 



