98 Bones of the Cranium. [Book IXi 



eight pieces of bone. At the fore part is placed the 

 os frontis ; at the back part the os occipitis ; at the 

 upper and fide parts the ofla parietalia ; at the under 

 and fide parts, the ofla temporalia ; in the fore part of 

 the bafe the os ethmoides ; in the middle of it the os 

 fphenoides. Thefe two latter bones are common to 

 the cranium and face. 



The os frontis is fo called from being the only bone 

 of the forehead, though it extends confiderably farther 

 upwards. It has fome refemblance in fhape to the 

 concha bivalvis, commcnly called the cockle. The 

 greater part of it is convex externally, and concave 

 internally, with a ferratcd circular edge. The upper 

 part of the os frontis, where it is connected to the pa- 

 rietal bones, is very fmooth and convex, but below it 

 has leveial inequalities, where it contributes confider- 

 ably to the formation of the cavities, in which the eyes 

 are lodged. In, the part of the os frontis which cor- 

 refponds with that part of the forehead immediately 

 above the eye- brows, the two tables of the bone fepa- 

 ratc by the external being protruded outwards, to form 

 two large cavities, called frontal finufes. Thefe ca- 

 vities communicate with the. external air by means of 

 the noie. The frontal bone ferves to fupport and pro- 

 tect the anteri r lobes of the brain. The falx of the 

 dura mater, of which I (hail have occafion to fpcak 

 more fully hereafter, is attached to a ridge or furrow 

 at the middle and internal part of this bone. The os 

 .frontis is pierced with fome frnail holes for the pafllige 

 of blood- veflels. 



Each of the two ofia parietalia is an irregular fquare, 

 its upper and fore fides being longer than that behind 

 or below. The inferior fide is a concave arch ; the 

 middle of ic receiving the upper and round part of the 

 temporal bo^ne. The external furface of each parietal 



bone 



