458 NERVOUS SYSTEM—GENERAL METHODS 



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 and it 

 allows of the most various after-treatments and methods of 

 staining. 



Several writers have insisted that for nervous tissue it should 

 not be acid, but some prefer it acid. See " Retina." For neuro- 

 fibrils it should be preferably neutral. To neutralise it, it is 

 generally sufficient to prepare its solutions with tap water, but 

 one may shake it with some calcium or magnesium carbonate. 

 Some authors prefer to neutralise it with ammonia. (See also 

 § 113.) 



The strength of the formalin solutions generally used for fixing 

 and hardening nerve-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 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 the fluid during the first days of fixation and 

 hardening. See further on this subject § 113. 



For anatomical work and for almost all histological processes, 

 10 parts of commercial formalin and 90 parts of 1 per cent, sodium 

 chloride form an excellent initial fixative. In spite of Nissl's 

 own opinion, good staining of Nissl bodies can be obtained in 

 human material after a few weeks' fixation by formol saline, 

 especially if the formalin solution is kept neutral. In older 

 material, washing the tissues in weak ammonia water before 

 imbedding greatly improves the staining. 



Stevenson has elaborated a method by which the bulk and 

 weight of the brain is kept constant {Arch. New. and Psych., vi, 

 1923, p. 763). It is found, however, that there is very little 

 change in the brain weight after formol saline fixation. 



Formalin can be associated with, or followed by, alcohol 

 (§ 113) or other reagents. See Fish {Trans. Am. Micr. Soc, 

 xvii, 1895, p. 319). 



Jore's fluid (see 8th edition) forms an excellent initial fixative. 



Parker and Floyd {Anat. Anz., xi, 1896, p. 156) advise for 



sheep's brains a mixture of 6 volumes of 95 per cent, alcohol and 



4 volumes of 2 per cent, formol. Brains may be kept in the 



mixture for months. 



Gerota (////. Monatschr. Anat., xiii, 1896, p. 124) treats foetal 

 brains by first injecting the vascular system with a 10 to 15 per 

 cent, solution of formol in 85 per cent, alcohol, and then bringing 

 the heads into 5 to 10 per cent, formalin ; after one or two days 



