ON THE USE OE EEAGENTS xxm 



and tie it with a bow. If the ligature be too tight it will cut 

 the vessel. Fill the syringe with the warm salt-solution : 

 remove the plug from the cannula : press the body of the 

 animal slightly, to remove some of the blood from its vessels, 

 and to get rid of any clot that may have formed close to the 

 cannula. Inject the salt-solution, to force the remaining blood 

 from the vessels before it can coagulate ; and wash or sponge 

 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 vers'd. 



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 in cold 

 water before dissecting it or putting it into spirit. 



VI. ON THE USE OE EEAGENTS. 



Reagents 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 sUces. For the mussel 

 a ^ per cent, solution of chromic acid ia 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. {See Appendix.) 



2. Staining with some colouring fluid renders the various 

 parts much more distinct, and is useful when the object is to 

 be examined microscopically, and especially when it has to be 

 cut into sections. 



