20 PRACTICAL MICROSCOPY. 



PREPARATION" OF TISSUES FOR CUTTING, ETC. 



We have already seen that most animal tissues are unsuitable for 

 the production of thin sections until hardened. 



It is also a fact, paradoxical though it may seem, that fresh tissues 

 do not present truthful appearances of structural elements. The old- 

 school histologists insisted upon the presentation of structures unal- 

 tered by chemical substances, while the modern worker has discarded 

 such tissue with very few exceptions. Many descriptions of structure 

 and growth, the result of study upon fresh material, have been proven 

 by later methods grossly inaccurate. 



It is impossible to remove tissues from the living animal and to sub- 

 ject them to microscopical observation without, at the same time, ex- 

 posing them to such radical changes of environment as to produce 

 structural alterations. Certain tissues, presenting in the living con- 

 dition stellate cells with the most delicate, though well-defined 

 branching processes, when removed from contact with the body, how- 

 ever expeditiously, afford no hint of anything resembling such ele- 

 ments, as they are quickly reduced to simple spherical outlines. 



In short, it is impossible to study fresh material, as such, without 

 constant danger of erroneous conclusions, as retrograde alterations of 

 structure commence with surprising rapidity the moment a part is 

 severed from the influences which control the complete organism. 



From what has been said we appreciate the necessity of agents 

 which, when applied to portions freshly removed from the animal, or 

 even before removal, shall instantly stop all physiological processes 

 and retain the elements in permanent fixity. 



Very much of the human structure which is available will be se- 

 cured only after functional activities have long ceased, and the structure 

 essentially altered. We are, therefore, compelled to resort to the use 

 of material from the lower animals in very many instances. 



ALCOHOL HARDENING. 



The tissue, whatever process may be in contemplation, having been 

 removed from the body as quickly after death as possible, ivithout 

 washing or allowing contact with water in any way, should, with a 

 sharp scalpel, be divided into small pieces. Of the more solid organs, 

 pieces one-half inch square by one-fourth inch thick will be suf- 

 ficiently small, and they will harden rapidly. The smaller the pieces 

 and the larger the quantity of hardening fluid the more quickly will 

 the process be completed. The volume of fluid should exceed that of 



