SECTION-CUTTING 213 



the application of some agent which brings about coagula- 

 tion of the component proteins 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 involv- 

 ing 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. 



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 harden- 

 ing*, but a properly hardened block is rarely firm enough to 

 permit of its being' cut into sections without further prepara- 

 tion. 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 that 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 arabic. It is then placed on the plate 

 of a microtome and frozen until the tissue assumes the con- 

 sistency 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 



