[ «36 ] 
» lower part of the.canula or barrel is a flat rim, projecting about 
a quarter of an-inch, on which the left hand, which grafps the 
canula, refts, to prevent it from flipping down on the part of the 
inftrument below it, whichis turned by the handle above; the 
crown has a center-pin, as.in the other inftruments, with a key to 
remove it when the fulcus.is deep enough to admit it to be taken 
away. Though with this the operation may with great eafe and 
fafety be entirely completed, yet to accommodate thofe who 
wifh to finith with the trephine, the upper part of the barrel or 
-eanula is made fquare to fit into a wooden handle; upon applying 
this handle, inftead of the winch, the inftrument is converted into 
a trephine; in this wooden handle is a fquare opening fitted to the 
-{quare part of the fpindle, ‘and faflened by the fame nut. 
Tr will be requifite to have two or three crowns of different 
“fizes, that the operator may choofe that which will beft fuit the 
circumftances of the cafe; as to the form of the crowns, modern 
practitioners have fo differed about them that every Surgeon mutt 
be left to his own choice. For my part I fhould prefer thofe 
as the beft and fafeft which are neither too conical nor too 
cylindrical, but between both extremes ; and, as * Mr. Chefelden 
has advifed, the cavity in the infide to enlarge in the fame 
manner the outfide does, to prevent the piece of bone to be taken 
eut from being wedged in the cavity, and to allow the crown or 
faw. to be inclined to one fide or to the other, as occafion may re- 
quire, during the operation. 
) Tuat 
* Chefelden’s Obfervations on Le Dran, page 447. 
