184 BACTERIOLOGICAL DIAGNOSIS. 



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 hap- 

 pens 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, &c.), 

 but they are rarely sufficiently thin for a proper demons- 

 tration 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 arabic. It is then 

 placed on the plate of a microtome 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 



