1 SECTION-CUTTING I7g 



processes of smearing the cut surfaces of tissues on clean slides 

 or cover-glasses, and treating the films thus obtained by the fixing 

 and staining methods previously described. If, for instance, we 

 have to search for tubercle bacilli in tuberculous glands, it is 

 usually sufficient to smear the cut surfaces of the glands on a slide, 

 dry, fix by heat, and stain by the same way as sputum is stained 

 for the tubercle bacillus. If anthrax bacilli were being looked for 

 in the liver or other organ removed post-mortem, the same 

 method of procedure would be adopted, except that Gram's 

 method of staining would be used. So also for typhoid bacilli in 

 the spleen, where the film would be stained with a simple stain 

 such as thionin or Loffler's methylene blue. 



It seems advisable, however, to give a short general account of 

 the processes involved in section-cutting, for they are by no means 

 difficult, and do not require very elaborate apparatus. Further, 

 the same methods of section-cutting are used for investigating 

 the nature of tumours, etc., and this is done already by many 

 practitioners, and should be done by still more. 



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 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 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 riot 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 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 d\ffi.c\AX.y freezing and embedding. 



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



