6 HISTOLOGIC.\L TECHXIC 



salt solution or blood serum. As a result they soon lose their char- 

 acteristic appearance and disintegrate. A proper killing or fixation 

 is therefore the first step in the j^ireparation of most tissues for micro- 

 scopic study, the object being to so preserve the tissues that they 

 retain as nearly as possible, and more or less permanently, the same 

 structure and relation which they had during life. Fresh tissues are 

 therefore essential for satisfactory results, as soon after death changes 

 set in which destroy the structure and relations of the tissue elements. 

 Tissues should also be handled as little as possible. Fixed tissue may 

 be placed in water, salt solution, alcohol, etc., in fact may be subjected 

 to many and varied manipulations which would destroy fresh tissues, 

 without disturbing the relation of the tissue elements. Fixation of 

 such a thin film of tissue as a blood smear may be accomplished by 

 heating. A blood smear or even small pieces of tissue may be fixed 

 by exposure to certain chemical vapors as, e.g., the vapor of formalin 

 or of osmic acid. Fixation is, however, usually accomplished by 

 means of chemicals in solution, the solution being known as a fixing 

 agent ox fixative. The tissue is immersed in the fixative and allowed 

 to remain there until fixation is complete. The pieces of tissue should 

 be small enough to permit rapid and thorough penetration, and large 

 quantities of the fixative should be used; at least ten times the bulk 

 of the tissue to be fixed. It may be necessary to change the fluid a 

 number of times, in order to keep it up to the proper strength. The 

 length of time required depends upon the character of the tissue and 

 upon the fixative used. In general it should be only long enough to 

 bring about the desired result, as prolonged immersion in the fixative 

 may make the tissue brittle or may interfere with the subsequent 

 treatment of the tissue, especially staining. 



Organs and even bodies may be fixed in toto by injecting the fixa- 

 tive through an artery and allowing it to escape through the veins. 

 After the injection, the whole specimen should be placed in a large 

 quantity of the same fixative. This method fills the entire vascular 

 system including the capillaries with the fixative, thus bringing the 

 latter into very prompt and close contact with the tissue elements. 

 The result is a very rapid and accurate fixation, which is especially 

 valuable where it is necessary to preserve the topographic relations of 

 various parts of an organ or of an entire body. 



The following are the fixatives in most common use: 



(i) Absolute Alcohol. — This is a rapid fixative and should be 

 used on small pieces of tissue. The time required is from six to 



