XX INTEODUCTION 



case of the lens, and passing a stout pin transversely through 

 the other end. The pin should be stuck upright into the 

 dissecting-board, with the lens over the part to be dissected : 

 focussing is effected by sliding the cork up amd down the pin. 

 The following rules for dissection should be carefully 

 observed : — 



1. Before commencing a dissection, fix the animal down 

 firmly to the dissecting-board or dish. 



2. In fixing an animal with pins, stick them in obliquely, 

 so that their heads do not get in the way or obscure the 

 dissection. 



3. Dissect under water, unless the animal is too large. 

 Change the water as soon as it gets dirty. A gentle stream of 

 water allowed to play upon the dissection is often a valuable 

 aid. 



4. Never cut away anything until you are quite certain 

 what it is you are removing. 



5. Put the part you are dissectiag slightly on the stretch. 

 This applies more particularly to blood-vessels, nerves, ducts, 

 and muscles. 



6. In cleaning blood-vessels, nerves, etc., dissect along 

 them, and not across them ; and avoid laying hold of them 

 with the forceps. 



7. The dissection is in many cases greatly faciHtated by 

 placing the specimen in spirit for a day or so before dissecting 

 it. In some cases the dissection may with advantage be per- 

 formed under spirit. 



8. Always keep your instruments clean and sharp. Be 

 careful not to blunt your fine scissors or scalpels by using 

 them for cutting hard parts. 



9. If you get in a muddle, stop, and wash the dissection 

 thoroughly under the tap before proceeding further. 



Successive slices cut fi:om an animal, or part of an animal, 

 with a razor are often exceedingly instructive ; this is especially 

 the case with the mussel and snail, and with the brains of the 

 rabbit and pigeon. The specimens must be previously har- 

 dened with spirit or other reagent, and the sHces should be 

 examined in water or spirit. 



