[ az ] 
the perforation is to be made on or near the upper part of the 
cranium. The affiftants muft be uncommonly attentive to keep 
the patient's head very fteady, for the leaft motion will throw 
the trepan out of its direction, which from its length it is liable 
to. The difficulty of keeping a patient quiet (unruly from the 
effets of the accident, or impatient under an operation which 
perhaps with reluctance he. has fubmitted to) is well known to 
every operator. 
‘To remove thofe impediments, fome operators, befides keeping 
one hand firmly preffed‘on the knob, place their forehead on it 
as direéted by * Dionis, or their chin as advifed by + Gareng- 
cott; thefe fituations, befides very much confining the operator, 
muft preclude him from feeing the progrefs of the faw, and 
when the furrow has been made int6 the fecond table of the fkull, 
and the refiftance againft the inftrument is become feeble, the 
bone may give way, and the faw, by being fuddenly prefled in, 
may injure the dura-mater and brain, and confequently death 
enfue. 
To prevent accidents of this kind, the older Surgeons guarded 
the faw with wings or fhoulders, and { Ambroife Paré tells us, 
he invented a ferula or ring, which he applied to the faw, with 
a ferew to fecure it from too fuddenly entering through the 
R 2 fkull; 
* Dionis’s Courfe of Operations, Demonftration 6th, pag. 284. 
+ Treite des‘Operations, Tom. 3, pag. 187, 188. ’ 
t Johnfton’s Tranflation of Ambroife Paré, Book 10, chap. 18, p. 245- 
