370 MICROTOMES AND SECTION KNIVES [Cn. XI 



by injection" is of great importance in the histology of animals which 

 are large enough to inject. 



If the animal is too small for injection or one wishes only a small 

 part of a larger animal, then the pieces for fixation should be small, 

 say one to three cubic centimeters. Often as for Flemming's fluid 

 ( 567) and for several others it is better to use pieces 2 to 5 cubic 

 millimeters in volume. 



Large, solid organs must be cut into several pieces if the whole 

 is needed. For hollow organs the cavity may be filled with the 

 fixer and the organ placed in a vessel of the same. 



The amount of fixer should be 10 to 50 times that of the piece of 

 tissue. 



Of the fixers given under " Preparation of Reagents," picric alcohol, 

 formaldehyde, and Zenker's fluid are suitable for almost every tissue 

 and organ. Formalin has the advantage of having strong penetra- 

 tion; hence it preserves whole animals fairly by immersing after filling 

 the abdominal and thoracic cavities. Formaldehyde is excellent 

 where a study of fat is in question, and it is much used as a fixer 

 where frozen sections are desired ( 609). Remember the necessity 

 of removing mercury from sections of tissues fixed with a mercuric 

 fixer (fig. 216-217). 



602. Mechanical preparation of tissues, etc., for microscopic 

 study. A limited number of objects in nature are small enough 

 and transparent enough, and a limited number of the parts of higher 

 animals are suitable for microscopic study without mechanical prepa- 

 ration except merely mounting them on a microscopic slide. Usu- 

 ally the parts of animals are so large and so opaque that the histologic 

 elements or cells and their arrangement in organs can only be satis- 

 factorily studied with a microscope after the tissue, organ, etc., have 

 been teased apart with needles, or sectioned into thin layers. 



MICROTOMES AND SECTION KNIVES 



603. The older histologists, those who laid the foundations and 

 whose understanding of the finer structure of the body was in many 

 ways superior to the knowledge possessed by workers at the present 

 time, did their mechanical preparation with needles and with sharp 



