l80 CLINICAL BACTERIOLOGY AND H.EMATOLOGY 



be carried out at home. The sections which it yields are usually 

 quite sufficient for purposes of histological research (the diagnosis 

 of tumours, etc.), but they are rarely sufficiently thin for a proper 

 demonstration of the bacteria which they may contain. The 

 sections are cut more easily by the freezing than by the paraffin 

 process, but they are decidedly more difficult to manipulate. 



In the freezing process the block or tissue after fixing and 

 hardening is dipped, or, better, soaked for some hours, in a thick 

 solution of gum arable. It is then placed on the plate of a micro- 

 tome and frozen until the tissue assumes the consistency of fairly 

 hard cheese, and can be cut into thin sections. 



The embedding process should be called the infiltration process ; 

 the tissue to be cut is infiltrated throughout with some firm 

 substance, and not merely embedded therein. Two embedding 

 materials are in general use — paraffin and celloidin. The latter 

 will not be described, as it is only necessary for special work, and 

 for ordinary purposes cannot compare with paraffin for beauty of 

 results and facility of application. 



In the paraffin process the tissue is infiltrated throughout with 

 hard paraffin (such as is used for the better varieties of paraffin 

 candles), so that every cell and every fibre is permeated and 

 supported on every side. To do this requires a number of 

 processes. It would be of no use to immerse the block of tissue 

 in the paraffin just as it is, for the paraffin would not wet it, much 

 less soak into it. The water is first removed, and this is done by 

 soaking the material in absolute alcohol. But alcohol does not 

 dissolve paraffin or mix with it ; it is therefore necessary to remove 

 it by means of some fluid which will mix with it on the one hand 

 and paraffin on the other. Of these there are many : xylol, 

 chloroform, benzine, cedar oil, and many more, are in use for 

 special purposes. Chloroform answers most purposes, and is to 

 be generally recommended, though ligroin is perhaps the best of 

 all. The block of tissue is now ready to be soaked in melted 

 paraffin ; it is kept in a bath of this substance until the chloro- 

 form has been entirely driven off and replaced by the paraffin. 

 The whole is then allowed to cool, is shaped into suitable blocks, 

 and is then ready for cutting. 



We shall now describe the processes in fuller detail. 



