44 



THE SKELETON 



the (ila.seiian fissure, which is subsequently encroaclied upon by the growth of the petrosal. As 

 the tympanic plate increases in size it joins the outer wall of the carotid canal and i)resents a 

 prominent lower edge, known as the vaginal process. 



The mastoid process becomes distinct about the first year, coincident with the obliteration 

 of the petro-s(iuamous suture. It increases in thickness by deposit from the periosteum. 

 Towards puberty, rarely earlier, the process becomes pneumatic, the air-cells being lined by deli- 



FiG. 53. — Tempor.\l Bone at the Sixth Year. 



AUDITORY MEATUS _> 

 OPENING IN THE TYMPANIC PLATE 

 GLASERIAN FISSURE 



{ 5l_ wormun bone in the parietal 



< ^ NOTCH 



cate mucous membrane. In old skulls the air-cells may extend into the jugular process of the 

 occipital bone. 



At birth the mastoid antram is relatively large and bounded externally by a thin i)late of 

 bone belonging to the squamosal. As the mastoid increases in thickness the antrum comes to lie 

 at a greater depth from the surface and becomes relatively smaller. 



THE PARIETAL 



The two parietals form a large portion of the vault and sides of the skull; they 

 are interposed Ijetween the frontal anteriorly and the occipital posteriorly. Each 

 parietal presents two surfaces, four borders, and four angles. The external surface 

 is convex and smooth : the convexit}', best marked in young bones, is greatest near 

 the centre, wliich is termed the parietal eminence. Crossing the middle of the bom- 

 are the two temporal ridges ; the lower is fre(|uently the better marked, and limits 

 the origin of the tcniporal muscle. The upper ridge is less constant, and gives attach- 

 ment to the temporal fascia. The internal surface is concave and marked Avith 

 <lepressions corresponding to the cerebral convolutions. Numerous vertical deej) 

 furrows for the branches of the middle meningeal artery radiate from the anterior 

 inferior angle and lower border of the bone. Along the superior margin of the 

 bone there is a groove which, when articulated with the opposite bone, forms a 

 furrow which receives the superior longitudinal sinus. In adult bones numerous 

 deep circular depressions for Pacchionian bodies are found near tliis groove. The 

 superior Itorder is deeply serrated for the opposite parietal, the union with which 

 forms the sagittal suture. The anterior and posterior borders are deeply serrated: 

 the anterior articulates with the frontal to form the coronal, and the jxjsterior with 

 the squamo-occipital to form the lambdoid sutures. The inferior border is 

 bevelled and overla])f)ed by the squamosal to form the squamous suture. Of the 

 angles, the anterior inferior is prolonged downwards and avticidates with the sum- 

 mit of the greater wing of the sphenoid. The posterior inferior angle articulates 

 with the mastoid portion of the petrosal; on its inner surface it lias a liorizontal 

 groove for lodging a portion of the lateral sinus. The superior angles present noth- 

 ing worthy of note. 



