CHAPTER XXXI. 401 



absorbs water from the tissues the latter shrink considerably, whilst 

 the alcohol loses its fixing and hardening properties through hydra- 

 tion. It has consequently to be changed soon for a fresh supply 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 investiga- 

 tions by Nissl's method (see 826), and for carrying out some of 

 Ramon y Cajal's reduced silver processes ( 827), 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. 



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

 histological technique by F. BLUM (Ztsohr. wiss Mikr., x, 1893, 

 p. 314) ; J. BLUM (Zool Anz., xvi, 1893, p. 434) ; HERMANN (Anat. 

 Anz., ix, 1893, p. 112) ; HOYER, jun. (Anat. Anz., ix, 1894, p. 236) ; 

 LACHI (Monit. Zool. Ital,.v, 1895, p. 15) and many others, its use 

 has been steadily increasing because of the many advantages it 

 offers. As a matter of fact it penetrates more quickly than solutions 

 of chromic salts, and even than alcohol ; it is not likely to over- 

 harden ; it allows of the most various after-treatments and methods 

 of staining, including neurofibril stains and Golgi's impregnation 

 method. 



Several writers have insisted that for nervous tissue it should not 

 be acid, but some prefer it acid. See " Retina." For neurofibrils it 

 should be preferably neutral. To neutralise it, it is generally suffi- 

 cient to prepare its solutions with spring water, but one may shake 

 it with magnesium or sodium or calcium carbonate. Some authors 

 prefer to neutralise with ammonia. (See also 108.) 



The strength of the formalin solutions generally used for fixing 

 and hardening nervous tissues varies considerably with the quality 

 of the material in hand, but particularly with the age of the subjects. 

 As a rule the more delicate the material and the younger the subject, 

 the weaker should be the formalin solutions to be employed at first. 

 Generally, however, one starts with a 3 or 5 per cent, solution in the 

 case of very soft tissues, gradually increasing the strength up to 

 10 or 12 per cent. An adult human encephalon can be very well 

 preserved in a 10 or 15 or 20 per cent, solution with two changes of 



M. 26 



