l6o CLINICAL BACTERIOLOGY AND HEMATOLOGY 



Slices of organs or tissues which are to be cut have first to be 

 fixed. The process of fixation consists essentially in the applica- 

 tion of some agent which brings about coagulation of the com- 

 ponent proteids with as little distortion of the morphological 

 elements as possible ; if this step were not carried out the 

 subsequent processes would be liable to cause alterations in the 

 shape, size, and appearance of the cells and fibres. There are 

 two chief methods of fixation, that involving the use of chemical 

 substances, and that involving the use of heat. The processes 

 which are used in fixing the tissues harden them at the same time ; 

 this is necessary, for fresh tissue would yield before the sharpest 

 knife, and could not be cut into thin sections. These processes are 

 always carried out, no matter what method of section cutting is to 

 be adopted, 



In cutting sections it is necessary that the material should be 

 sufficiently firm and homogeneous in consistency. The former is 

 secured to some extent by the process of hardening, but a 

 properly hardened block is rarely firm enough to permit of its 

 being cut into sections without further preparation. Further, it 

 almost invariably happens that some parts of the material are 

 firmer or harder than others ; and if such a substance were cut 

 the harder parts might be sufficiently firm, whilst the softer parts 

 would simply crumble before the knife. There are two methods 

 of overcoming this difficulty — freezing and embedding. 



The freezing process is very simple, and it is one which can easily 

 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 



