430 AMOUNT OF A BIOLOGICAL SAMPLE 



In practice, the inicrotome without its tissue freezing block is 

 placed in the cabinet, the Dry-Ice chamber is filled with small 

 pieces, the blower is started, and after about 45 min. the tempera- 

 ture is at —20 ±0.5°. A cylinder of tissue is frozen to the block 

 using carbon dioxide, and the block is then brought inside the 

 cabinet through a removable window (C). After the block is 

 fastened to the microtome, the knife is set and cutting is begun. 

 When not in use, the microtome is stored in a large desiccator with 

 sulfuric acid. 



The sections are prevented from curling on the knife edge by the 

 arrangement shown in Figures 153 and 154. A plate of glass (A) is 

 held at a distance of 50 /x from the knife by two strips of cellophane 

 tape (B). The glass plate can be moved on the hinge (C), and the 

 spring (D) holds the plate against the cellophane strips. The upper 

 edge of the plate coincides with the knife edge ; screws (F) assist in 

 the adjustment. The microtome is operated at constant speed and 

 without stopping for each series of sections to obtain uniform cut- 

 ting. After the glass plate is swung back, the first section in each 

 series is discarded since its thickness is different from that of the 

 others, and the sections to be used are transferred to slides by a thin 

 glass rod or fine brush. 



B. MICROSCOPIC EXAMINATION AND CHEMICAL 

 ANALYSIS OF THE SAME TISSUE SECTION 



In the usual procedure, alternate sections are employed for chem- 

 ical determination and histological examination in order to corre- 

 late the analysis with the morphological constitution of the tissue. 

 This procedure is suitable, provided the histology of the section 

 analyzed and that of the adjacent section studied microscopically 

 are essentially the same. When this is not true, as in the case of 

 retinal tissue which has histological changes every 40 to 50 fx, it is 

 necessary to carry out the analysis and the microscopy on the same 

 section. A method for accomplishing this was worked out by An- 

 finsen et al. (1942), who first stained and examined the section and 

 then used it for chemical measurement. 



