NERVOUS SYSTEM— GENERAL METHODS 455 



in view and the subsequent treatment to whieh the tissues are 

 to be submitted. In the case of animals it is a good practice to 

 warm the fixing fluid to body-temperature before injecting it, 

 and, whenever possible, to wash out the blood by first injecting 

 physiological solution as suggested by Mann. The injection can 

 be carried out through the carotids if the fixation is to be limited 

 to the encephalon, and through the aorta if it is desired to fix the 

 spinal cord too. The above applies to higher vertebrates and 

 particularly to mammals ; in the case of lower vertebrates, 

 fixation by injection has not, as a rule, the same importance, and 

 one must have recourse to special methods. 



See on this subject Golgi, op. cit., § 1027 ; Gerota, § 983 ; De 

 QuERVAiN, Virchoiv''s Arch., cxxxiii, 1893, p. 515 ; Mann, Ztschr. wiss. 

 Mikr., xi, 1894, p. 482 ; Strong, Anat. Anz., xi, 1896, p. 655 ; Joiirn. 

 Comp. Neurol., xiii, 1903, p. 291 ; McFarland, Journ. App. Micr., ii, 

 1899, p. 541 ; or Ztschr. wiss. Mikr., xvii, 1900, pp. 39, 40. 



979. Hardening by Freezing. This phrase has often given rise 

 to confusion and should, therefore, be clearly understood. One 

 can harden by freezing either fresh tissues, or material already 

 fixed and consequently also a little hardened. In the first instance 

 small pieces of fresh tissue, immediately after removal and with- 

 out any previous treatment, are hardened on a freezing microtome. 

 The sections are generally floated on to water, and immediately 

 afterwards treated for a minute on the slide with a 0-25 per cent, 

 solution of osmic acid ; or otherwise treated according to the 

 object of one's investigation. Chilling the knife after the method 

 of ScnvTz-BnAVss (Virchow's Arch., 1929, cclxxiii, p. 1) is particu- 

 larly useful. Since Brodmann (Journ. Psychol, u. Neurol., ii, 

 1903-04, p. 211) has shown that formalin material can be used 

 even for investigations by polarised light, section of unfixed tissue 

 is rarely needed. (See also p. 895.) 



The hardening by freezing of already fixed material may be 

 also attended with some difficulty, but this will be easily over- 

 come if pieces are relatively small, the fixing agent properly 

 washed away, and one has, eventually, recourse to one or other 

 of the processes described in § 197. Nervous tissue well fixed in 

 formalin is very suitable for frozen sectioning. Care must be 

 taken that the material is not too hard when cut. For a descrip- 

 tion of the freezing technique refer to Chapter XII. 



The hardening and section cutting by the freezing method of 

 very large pieces require special apparatus and special methods, 

 for which see Nageotte, C. R. Soc. Biol., Ixvii, 1909, \). 542. 

 The large freezing box made for Reichert's sledge microtome is 

 very suitable for this purpose. 



980. Hardening by Reagents. If large pieces of nervous 

 tissue are to be hardened, it is necessary to take special pre- 

 cautions in order to prevent them from being deformed by their 



