ANATOMICAL TECHNIQUE 451 



sides. It is usually best to allow the material to remain for twenty- 

 four hours in the first grade of collodion or celloidin. The bottle is 

 then cooled in the air or more quickly by cold water. The fluid is 

 thrown away because of the accumulation of extractives from the 

 material. Four per cent of celloidin is then poured on, and the 

 bottle wired again for a twelve-hour immersion in the warm bath. 

 The second solution is returned after cooling to its proper stock 

 bottle, and the successive grades of dissolved nitrocellulose are used 

 respectively until 16 per cent is reached. At this stage chips of 

 nitrocellulose previously dissolved and then dried are used at inter- 

 vals and in a small amount at a time further to thicken the highest 

 grade of solution. The bottle is returned to the bath with each 

 addition of dry nitrocellulose. If the process has been carried 

 on with proper precautions, the material has becone infiltrated 

 without shrinkage. In the case of soft material which is at the 

 same time rather impervious it is often an advantage thoroughly 

 to prick the pieces with a very fine needle. The pricks are not 

 usually near enough to injure the appearance of the sections which 

 are cut later, if all are made in the same plane, naturally not that 

 of greatest importance from the structural standpoint. 



After a sufficient degree of thickening has been attained in the 

 manner described in the foregoing paragraph, the objects, each 

 surrounded by a coating of nitrocellulose, are removed to chloro- 

 form for hardening. The chloroform should be liberal in amount 

 to insure thorough induration of the nitrocellulose, and the mate- 

 rial should be kept in the reagent for at least twelve hours. Sub- 

 sequently it is transferred to a mixture of equal parts of alcohol 

 and glycerin, in which it may remain until needed for cutting. 

 Objects stored in this way seem to maintain their properties indefi- 

 nitely and are certainly still useful after as long an interval as twenty 

 years. The alcohol should be prevented from evaporating by 

 occasional renewals. 



SECTIONING OF MATERIAL 



Only in the rarest instances is it desirable to cut material free- 

 hand with a sectioning razor. If uniformly good results are sought, 

 the microtome should be used. There are many types of micro- 

 tomes, but those with the simplest mechanisms and the greatest 



