140 THE SKELETON. 



Angles. The anterior superior, thin and pointed, corresponds with that por- 

 tion of the skull which in the foetus is membranous, and is called the anterior 

 fontanelle. The anterior inferior angle is thin and lengthened, being received in 

 the interval between the great wing of the sphenoid and the frontal. This point 

 will be found about one inch behind the upper and outer angle of the orbit. 

 Its inner surface is marked by a deep groove, sometimes a canal, for the an- 

 terior branch of the middle meningeal artery. The posterior superior angle cor- 

 responds with the junction of the sagittal and lambdoid sutures. In the foetus 

 this part of the skull is membranous, and is called the posterior fontanelle. The 

 posterior inferior angle articulates with the mastoid portion of the temporal bone, 

 and generally presents on its inner surface a broad shallow groove for lodging 

 part of the lateral sinus. 



Development. The parietal bone is formed in membrane, being developed by 

 one centre, which corresponds with the parietal eminence, and makes its first 

 appearance about the fifth or sixth week of fcetal life. Ossification gradually 

 extends from the centre to the circumference of the bone. The angles are con- 

 sequently the parts last formed, and it is in their situation that the fontanelles 

 exist, previous to the completion of the growth of the bone. 



Articulations. With five bones ; the opposite parietal, the occipital, frontal, 

 temporal, and sphenoid. 



Attachment of Muscles. One only, the Temporal. 



THE FRONTAL BONE. 



The Frontal Bone, which resembles a cockle-shell in form, consists of two 

 portions a vertical or frontal portion, situated at the anterior part of the cra- 

 nium, forming the forehead; and a horizontal or orbito-nasal portion, which 

 enters into the formation of the roof of the orbits and nose. 



Vertical Portion. External Surface (Fig. 94). In the median line, traversing 

 the bone from the upper to the lower part, is occasionally seen a slightly ele- 

 vated ridge, and in young subjects a suture, which represents the line of union 

 of the two lateral halves of which the bone consists at an early period of life. 

 In the adult, this suture is usually obliterated, and the bone forms one piece. 

 Traces of the obliterated suture are, however, generally perceptible at the lower 

 part. On either side of this ridge, a little below the centre of the bone, is a 

 rounded eminence, the frontal eminence. These eminences vary in size in 

 different individuals, and are occasionally unsymmetrical in the same subject. 

 They are especially prominent in cases of well-marked cerebral development. 

 The whole surface of the bone above this part is smooth, and covered by the 

 aponeurosis of the Occipito-frontalis muscle. Below the frontal eminence, and 

 separated from it by a slight groove, is the superciliary ridge, broad internally 

 where it is continuous with the nasal eminence, but less distinct as it arches 

 outwards. These ridges are caused by the projection outwards of the frontal 

 sinuses, and give attachment to the Orbicularis Palpebrarum and Corrugator 

 Supercilii. 1 Beneath the superciliary ridge is the supra-orbital arch, a curved and 

 prominent margin, which forms the upper boundary of the orbit, and separates 

 the vertical from the horizontal portion of the bone. The outer part of the 

 arch is sharp and prominent, affording to the eye, in that situation, considerable 

 protection from injury ; the inner part is less prominent. At the inner third 

 of this arch is a notch, sometimes converted into a foramen by a bony process, 



1 Some confusion is occasioned to students commencing the study of anatomy, by the name 

 " sinuses" having been given to two perfectly different kinds of spaces connected with the skull. 

 It may be as well, therefore, to state here, at the outset, that the " sinuses" on the interior of the 

 cranium, marked by grooves on the inner surface of the bones, are venous channels along which 

 the blood runs in its passage back from the brain, while the " sinuses" on the outside of tha 

 cranium (the frontal, ethmoidal, sphenoid, and maxillary) are hollow spaces iu the bones them- 

 selves, which communicate with the nostrils, and contain air. 



