20 STUDENTS HISTOLOGY 



fluids, preferably formaldehyde, may be taken. Small pieces that 

 have been hardened for a few hours in formaldehyde may be cut 

 very thin after freezing, and this plan is most useful in pathological 

 work. Sections of fresh tissues may be examined with the micro- 

 scope directly, or they may be placed in formaldehyde, and after- 

 wards stained and mounted as directed in the succeeding pages. 

 Sections of hardened tissues may be stained and mounted in 

 balsam. 



Freezing of the specimen is done upon a metal box or plate, 

 which may be attached to the ordinary microtome. Microtomes 

 designed especially for freezing are also made (Fig. 14). Freezing 

 is best accomplished by the escaping stream of carbon dioxide gas, 

 derived from a cylinder of the compressed gas. An ether -spray 

 may be used for the same purpose, or the water running from a 

 salt and ice freezing mixture through a small metal case which may 

 be placed on the microtome. The pieces of tissue should be not 

 more than five millimeters thick. 



FIXING OR HARDENING FLUIDS 



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 struc- 

 tures unaltered by chemical substances, while the modern worker 

 has discarded such tissue with very few exceptions. Many descrip- 

 tions for structure and growth, the result of study upon fresh 

 material, have been proved by later methods grossly inaccurate. 



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

 subject them to microscopical observation without, at the same 

 time, exposing them to such radical changes of environment as to 

 produce structural alterations. Certain tissues presenting in the 

 living condition stellate cells with the most delicate, though well- 

 defined, branching processes, when removed from contact with the 

 body, however expeditiously, afford no hint of anything resem- 

 bling such elements, as they are quickly reduced to simple 

 spherical outlines. 



In short, it is impossible to study fresh material, as such, with- 

 out constant danger of erroneous conclusions, as retrograde altera- 

 tions of structure commence with surprising rapidity the moment 



