NERVOUS SYSTEM— GENERAL METHODS 457 



Marie's method of fixing and hardening in situ is highly 

 recommended ; for its indications and contra-indications, see 

 Sainton and Kattwinkel {Deutsche Arch. kiln. Med., Ix, 1898, 

 p. 548) and Pfister {Neurol. CentrbL, xvii, 1898, p. 643). 



981. The Reagents to be Employed. As in the case of the 

 fixation by injection one should bear in mind that the preserva- 

 tion 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 unsuit- 

 able for anatomical methods. (See § 977.) On the other hand, 

 material collected and prepared for cyto-architectural or fibro- 

 architectural 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. 



982. Alcohol. It is generally employed in the strength of 

 94 to 96 per cent., penetrates well and hardens quickly ; but 

 as it rapidly absorbs water from the tissues the latter shrink 

 considerably, while the alcohol loses its fixing and hardening 

 properties through hydration. It has consequently to be changed 

 soon and used in quantities exceptionally large in proportion to 

 the size of the pieces, which ought to be neither too small nor too 

 large. For this reason one seldom hardens in alcohol voluminous 

 organs, and its use has become on the whole very restricted. 

 Alcohol, however, remains the principal fixing and hardening 

 reagent for cytological investigations by Nissl's method (see 

 § 1000), and for carrying out some of Ramon y Cajal's reduced 

 silver processes, its shrinking influence being counteracted by 

 having recourse for the first fixation to weaker dilutions of alcohol 

 (60 to 70 per cent.) to be raised gradually up to 95 or 96 per cent, 

 within the first nine to twelve hours, and to be changed once or 

 twice or more often in the next few days. 



983. Formalin. Since the time when it was introduced into 

 histological technique by F. Blum {Ztschr. wiss. Mikr., x, 1893, 

 p. 314) ; J. Blum {Zool. Anz., xvi, 1893, p. 450) ; Hermann 

 {Anat. Anz., ix, 1893-94, p. 112) ; Hoyer, jun. {Anat. Anz., Verf. 

 Anat. Ses., ix, 1894, p. 236 ) ; Lachi {Monit. Zool. Ital, v, 1895, 

 p. 15) and many others, its use has been steadily increasing 



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

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

 nervous system a 5, 10 or 20 per cent, solution of formaUn 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. 



