398 NERVOUS SYSTEM GENERAL METHODS. 



The choice of the fluid to be injected depends upon the object in 

 view and the subsequent treatment to which 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., in 880 ; GEROTA, 811 ; DE 

 QUERVAIN, Virchow's Arch., cxxxiii, 1893, p. 481 ; MANN, Ztschr. wiss. 

 Milcr., xi, 1894, p. 482 ; STRONG, Anat. Anz., xi, 1886, p. 655 ; Journ. 

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

 1899, p. 541. 



Hardening. 



807. Hardening by the Freezing Method. 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 without 

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

 sections are generally floated on to water, and immediately after- 

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

 tion of osmic acid ; or otherwise treated according to the object of 

 one's investigation. In this case the ether freezing method should 

 be preferred, bearing in mind, however, that there is considerable 

 difficulty in obtaining sufficiently good sections, and that the 

 results attainable are very limited particularly since BRODMANN 

 (Journ.Psychol. Neurol., ii, 1903 4, p. 211) has shown that formalin 

 material can be used even for investigations by polarised light. (See 

 also p. 361.) 



The hardening by freezing of already fixed material may be also 

 attended with some difficulty, but this will be easily overcome 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 183. Material fixed in formalin, however, does not, 

 as a rule, require any soaking in gum, or syrup, or the like, and is 

 easily cut if the formalin has been washed away. In this case the 

 C0 2 freezing microtome is in my opinion to be preferred. 



