eel 
P 
THE OCCIPITAL BONE. 105 
sphenoidalis), pointed and prominent, articulates with the great wine of the 
sphenoid. It is wedged into the angle formed by the union of that bone with the 
frontal, and is bevelled at the expense of its inner table anteriorly, whilst inferiorly 
it is thinned at the expense of its outer table. The posterior inferior angle (angulus 
. . . . . . 5 
mastoideus) is a truncated angle lying between the inferior and posterior borders. 
It is deeply serrated, and articulates with the mastoid process of the temporal bone. 
Not unfrequently there is a channel in this suture which transmits an emissary vein. 
Connexions.—The parietal bone articulates with its fellow, with the frontal, occipital 
mastoid and squamous temporal, and with the sphenoid. 
Architecture.—Thin towards its lower part, where it enters into the formation of the 
temporal fossa, it is thickest along the superior border and in the neighbourhood of the posterior 
superior angle. 
Variations.—A number of cases have been recorded in which the parietal is divided into an 
upper and lower part by an antero-posterior suture parallel to the sagittal suture. Coraini 
(Att. d. XI. Congr. Med. Internaz. Roma, 1894, vol. v.) records a case in which the parietal was in- 
completely divided into an anterior and posterior part by a vertical suture. The parietal 
Depressions for Pacchionian bodies 
ae foramen 
Groove for 
superior longi- 
tudinal sinus 
~ 
\ - 
as 
Anterior inferior angle Grooves for middle 
meningeal artery 
Groove for lateral sinus 
Fra. 81.—RIGHT PARIETAL BONE (Inner Surface). 
foramina vary greatly in size, and to some extent in position. They are sometimes absent on one 
or other side, or both. They correspond in position to the sagittal fontanelle. Sometimes the 
ossification of this fontanelle is incomplete and a small transverse fissure remains. The parietal 
foramen represents the patent external extremity of this fissure after its edges have coalesced. 
_. Ossification.—Ossification takes place in membrane by the deposition of earthy 
matter, the centre for which, most probably formed by the coalescence of two nuclei, 
appears over the parietal eminence about the sixth or seventh week of foetal life ; from this, 
it spreads in a radial manner towards the edges of the bone, where, however, the mem- 
branous condition still for some time persists constituting the fontanelles. These corre- 
spond in position to the angles of the bone. Ossification is also somewhat delayed in 
the region of the parietal foramina, constituting what is known as the sagittal fontanelle, 
a membranous interval which is not unfrequently apparent, even at birth. 
THE OCCIPITAL BONE. 
The occipital bone (os occipitale), placed at the back and lower part of the 
cranium, consists of three parts, arranged around a large oval hole, called the 
