PRESERVATION OF SPECIMENS IN PLASTICS 57 



integuments should be punctured in two or more places with a dissecting 

 ! needle or other sharp implement before immersion to facilitate entrance 

 of the monomer into the body cavities. In the case of large specimens it 

 i might also be advisable to inject monomer, or partial polymer sirup, 

 into the principal cavities through the use of a syringe. 



Fragile specimens, and also those that are already permeated with 

 monomer as the result of being transferred from a liquid dehydrating 

 medium, should be allowed to stand completely covered with monomer 

 . for at least a day, and during this time it is best to place the vessels con- 

 taining the specimens in a refrigerator where a temperature of about 5° C. 

 (41° F.) is maintained to guard against premature polymerization. If 

 desired, most specimens may be kept for a considerable length of time 

 in monomer, provided they are stored at a temperature low enough to 

 retard polymerization. Specimens that contain extractable pigments, 

 however, should be removed from the monomer bath as soon as full 

 saturation has been attained and embedded as promptly as possible by 

 means of the quick-setting, viscous, partial-polymer sirup. 



Specimens that are porous or of a generally permeable nature can 



usually be impregnated to the best advantage under partial vacuum, if 



they are sufficiently strong to withstand the effect of the escaping air 



bubbles. Such specimens should be immersed in monomer, or in some cases 



in thin partial-polymer sirup, and the container, after being loosely 



' covered with a flat plate of glass or metal, should be placed in a vacuum 



I- desiccator. An aspirator rather than an oil-type vacuum pump should be 



J: used to evacuate the desiccator, because the methacrylate vapor would 



tend to dilute the oil in the latter and eventually impair its operation. 



A suitable trap should be interposed between the aspirator and the 



»j vacuum chamber to serve as a safeguard against entrance of water in 



1 case of a temporary reduction of pressure in the water-service lines dur- 



I ing evacuation. A vacuum gauge or manometer in the system is also 



I desirable so that the degree of vacuum can be readily noted at all times. 



| It is always best to keep the vacuum-chamber stopcock closed until after 



the water has been turned on full force through the aspirator, and likewise 



i to close the stopcock before the water is shut off. 



In subjecting the specimen to vacuum, a negative pressure of between 

 I 25 and 29 inches of mercury will usually be found to be suitable. It is 

 I important, however, to watch the specimen closely and to control the 

 ■ vacuum so that violent bubbling is avoided. The removal of entrapped 

 |i air is usually facilitated by alternately evacuating the chamber and then 

 li allowing air to return into it so as to produce a pumping action on the 

 I specimen. It is advisable to continue evacuation in this manner until no 

 I more air bubbles are given off, or at least until the specimen tends to 

 sink into the monomer under normal atmospheric pressure. Whenever 

 air is allowed to re-enter the chamber it should be admitted slowly in 

 1 order to avoid strains which might cause breakage of the glass. It is also 

 if advisable to watch the monomer for possible thickening due to premature 

 I polymerization, and to replace it with new monomer before it becomes 

 : unworkably viscous. 



As soon as the specimen has become completely saturated with 

 < monomer, it is ready for embedment. If the embedding operation is not 

 i; to be carried out at once, the specimen should be kept immersed in the 

 liquid methacrylate and stored in a cold place. 



