NEED AND ACTION OF REAGENTS. 



GENERAL CONSIDERATIOWS AS TO THE NECESSITY FOR REAGENTS AND 

 THEIR PROBABLE ACTION UPON THE TISSUE. 



Fixation. — The term "fixation" may be briefly defined as a 

 process by which the tissue is quickly killed and its structure 

 rendered permanent. This definition implies the use of fresh 

 tissue. -The structural elements of the tissue should be in their 

 normal or living attitude at the time of fixation, and the fixing 

 reagent should catch and maintain this attitude. To accomplish 

 this, the reagent must render the elements rigid, insoluble, and 

 able to resist without change the action of the reagents to which 

 it may be necessary to subject the tissue during the manipula- 

 tion following fixation. 



While it is very essential for the student to examine tissues 

 in the fresh state, so as to become acquainted with their normal 

 appearances under the microscope, yet such preparations are not 

 only not permanent, but also, after a short time, post-mortem 

 changes set in, and the beginning dissolution gives appearances 

 far from normal. Also, in mounts of fresh tissue, the structural 

 elements are often so homogeneous in their behavior toward 

 light 'that some special procedure must be resorted to which 

 will result in an optical differentiation of the component parts. 

 In some cases the action of the fixing agent directly results in 

 this differentiation of the tissue elements by altering their 

 indices of refraction. In most cases, however, it is attained by 

 the use of certain stains. In nearly all the special differential 

 methods the selective action of these stains depends to a great 

 degree upon the previous action of the fixing reagent upon the 

 tissue. 



A good fixing agent, therefore, must possess at least three 

 qualities : It must have the power to penetrate quickly, and 

 thus fix the tissue elements before post-mortem changes have 



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