ON REAGENTS xxi 



the blood away. If the arteries are being injected, the vena 

 cava and portal veins should be cut open to allow free escape 

 of the blood, and vice versa. 



Mix the plaster of Paris in the mortar, stirring in the 

 colour, and making the plaster thin. Strain it rapidly through 

 the muslin, and inject immediately with the syringe. When 

 the vessels appear to be well injected, remove the syringe and 

 insert the glass plug, and wash the animal to get rid of blood 

 and any injection that may have escaped. 



Allow the animal to remain two or three hours before 

 dissecting it or putting it' into spirit. 



VI. ON THE USE OF EEAGENTS. 



Eeagents are used for hardening, staining, and preserving 

 specimens. Those required for general use are but few in 

 number, and directions for their preparation will be found in 

 the appendix. 



1. Hardening is necessary in the case of soft animals or 

 tissues of which it is proposed to cut slices. For the mussel 

 a \ per cent, solution of chromic acid in water serves well. 

 For the brain of a rabbit or pigeon ordinary methylated spirit 

 is very convenient and effective. The specimen must be left 

 in the hardening fluid two or three days, and a large bulk of 

 the fluid must be used, or else it must be frequently changed. 



Other methods are used when the specimen is to be cut 

 into microscopical sections. 



2. Staining with some colouring fluid renders the various 

 parts much more distinct, and is especially useful when the 

 object is to be examined microscopically, and especially when 

 it has to be cut into sections. 



Magenta is useful for staining fresh specimens, but the 

 stain is not permanent. 



Carmine is perhaps the most useful of all stains. It 

 affects different cells and parts of cells differently, and the 

 stain is permanent. The most useful preparation of it is 

 Grenacher's borax-carmine : this should be used warm, and 

 the specimens after removal from it should be placed in acid- 

 alcohol for a few minutes or hours, according to their size. 



