l82 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



The other steps are the same as those which are used if other 

 methods of fixation have been adopted. 



2. Formahn. This should be used in a 5 per cent, solution in 

 normal saline solution (o-g per cent.). It yields very good results, 

 and is perhaps the fluid which can be most warmly recommended 

 to a practitioner who is going to send his material to a public 

 laboratory.* The fluid has very great powers of penetratioui and 

 the slices may be much thicker than we have recommended. 

 The one objection to the fluid is that it interferes somewhat with 

 the way in which the sections stain. 



3. Alcohol is a very good fixing fluid. When it is used the 

 blocks should be cut small and placed at once in methylated 

 spirit diluted with an equal quantity of water. 



SECTION-CUTTING BY THE FREEZING METHOD 



Sections which are prepared by the freezing method are rarely 

 as thin as those prepared by one or other of the infiltration 

 processes, but are prepared very rapidly, and are often sufficient 

 for diagnostic purposes, where rapidity is the first consideration. 



The blocks of tissue must be hardened before being cut, any 

 of the above fluids being applicable; where alcohol is used it 

 must be washed out in water, as it will not freeze. Where more 

 rapid work is required the best method is a modification of the old 

 boiling process, as revived by Mr. Strangeways. The slices of 

 tissue from which sections are to be cut are thrown at once into 

 boiling water, and allowed to boil vigorously for two or three 

 minutes ; the water must be actually boiling when the tissues are 

 added, and the bulk used should be large as compared with the 

 block. The tissues are then rapidly cooled by being thrown into 

 cold water, and are then ready for cutting. The outer surface of 

 the block should be rejected. 



This method of fixation leads to a little distortion of the tissues 

 and alters any blood which they may contain, but it is very good 

 for diagnosing tumours. It is invaluable in the post-mortem 

 room, and for diagnosis of the nature of a tumour during opera- 

 tion. In skilful hands a section may be cut, stained, mounted, 

 and a diagnosis made in ten minutes ; or if no process of fixation 



* Formalin should not be used for tissues which are to be searched for the 

 tubercle bacillus, as it prevents the decolorizing action of the acid. 



