402 HISTOLOGICAL TECHNIQUE. 



better for the fixation of some tissues than for others. As a rule, 

 those solutions that most perfectly preserve the finer intracellular 

 details of structure have very little power of penetrating masses 

 of tissue. They can, therefore, only be employed when very small 

 bits of tissue are to be fixed. Other fixing-solutions penetrate 

 much better, but fail to fix the most delicate structures, which may 

 undergo changes before they are preserved. It follows that the 

 choice of the method of fixation must in each case depend upon 

 the object to be attained. 



The removal of water from the fixed tissues is accomplished by 

 means of alcohol. The fixing-agents are nearly all aqueous solu- 

 tions, and while they increase the consistency of the tissues to a 

 certain extent, they do not usually render them sufficiently firm for 

 the preparation of thin and uniform sections. If the water in the 

 tissues be replaced by alcohol, a greater and more uniform con- 

 sistency is obtained, and the tissues are also partly prepared for 

 impregnation with an embedding-material (collodion or paraffin) 

 should that be necessary for section-cutting. 



After sections of fixed tissues have been obtained they usually 

 require staining before they can be profitably studied. The chief 

 reason for this will appear in the following explanation : 



When a specimen is examined under the microscope differences 

 in structure among the colorless elements of the specimen may be 

 seen, or differences in color between the different elements may be 

 perceptible. We may, then, distinguish between a " structure- 

 picture/' due to differences that are not those of color, and a " color- 

 picture," due solely to such differences. The manner in Avhich the 

 latter is produced is, perhaps, self-evident. The structure-picture 

 is the result mainly of differences in refraction due to the various 

 densities of different parts of the specimen. But the processes of 

 fixation and hardening have for their purpose the rendering of the 

 tissues of a relatively uniform density. They must, in consequence, 

 tend to obliterate the details of the structure-picture which the 

 sections yield when viewed under the microscope. For this reason 

 the sections are stained, which converts the structure-picture into a 

 color-picture. 



The substances composing the tissues have various affinities for 

 dyes, and it is possible to take advantage of this in staining sec- 

 tions, so that structures of the same nature shall receive one color, 

 while those of different composition shall be dyed of a different 



