FIXATION 



The main objects of fixation are: 



(a) To kill the cells suddenly and uniformly so that they retain, 

 as near as possible, the same appearance which they possessed in 

 life. 



(b) To preserve the tissues, cells, etc., by the inhibition of 

 putrifactive and autolytic changes. 



(c) To set and hold intra-cellular bodies, cells, etc., by precipi- 

 tation in the positions which they occupied in life, thereby facili- 

 tating the closest possible study of the histology and cytology of 

 the cells. 



(d) To facilitate differentiation in the refractive indices of cer- 

 tain cell elements which would otherwise be invisible owing to the 

 exceedingly narrow margin betw^een the refractive index of one 

 type of cell and that of other types. 



(e) To render the cells and tissue constituents resistant to the 

 subsequent processes such as dehydration, clearing, embedding, 

 staining, etc., prior to their examination under the microscope. 



(/) To facilitate proper staining of tissues. Here it should be 

 mentioned that some fixatives act as mordants while others act as 

 inhibitors for certain stains, and it is, therefore, of considerable 

 importance that a suitable fixative should be employed for a 

 particular staining technique or a particular staining technique 

 should be chosen to suit material which has already been treated 

 with a particular fixative. Recommendations as to suitable fixa- 

 tives are given for most, if not all, of the staining techniques des- 

 cribed in this book. 



It is essential, if good results are to be achieved, that tissues 

 should be removed from the body with the least possible delay 

 and fixed immediately. Bacteria, protozoa and other unicellular 

 organisms should be living at the instant of fixation. The follow- 

 ing points must be observed in order to avoid failure, waste of 

 time, effort and materials : 



