XX INTRODUCTION 



III. ON THE METHODS OF KILLING ANIMALS. 



The method of killing is of considerable importance, and 

 careful attention should be paid to the directions given at the 

 commencement of each chapter. 



Two methods of killing animals are in general use, and 

 the one to be chosen depends upon whether the specimen is 

 to be (1) dissected and examined temporarily, or (2) con- 

 verted into permanent microscopical preparations. 



For the first purpose chloroform is useful. Three or four 

 minutes' immersion is sufficient for small animals, after 

 which the animal should be washed in water. For frogs and 

 rabbits at least fifteen minutes are needed. This answers well 

 for the leech and earthworm. For killing crayfish boiling 

 water may be employed, and snails die well expanded if 

 drowned in warm water. 



For the second purpose a reagent is employed which not 

 only kills the tissues, but penetrates and ' fixes ' them, so 

 that they are but slightly affected by subsequent treatment. 

 There are three chief fixatives : osmic acid, corrosive subli- 

 mate, and Miiller's fluid, the uses of which are given in the 

 Appendix. Strong alcohol is the simplest and most gene- 

 rally applicable, but does not give the best results for histo- 

 logical work. For Protozoa, osmic acid or the vapour of 

 glacial acetic acid is useful ; for Hydra, Perenyi's fluid. For 

 larger animals a mixture of 90 parts concentrated corrosive 

 sublimate solution and 10 parts glacial acetic acid is a good 

 reagent. The specimen must be subsequently washed in 

 alcohol. 



IV. ON DISSECTION. 



The object of dissection is to separate the several organs 

 from each other, so far as is necessary to define their boun- 

 daries and display their relations to each other. It consists 

 mainly in the removal of the connective tissue which binds 

 the parts together and obscures their outlines. 

 The necessary instruments are the following : 

 1. Two or three scalpels, of different sizes. 



