80 



SPECIAL ANATOMY OF THE SKELETON 



in the median line by the ethmoidal notch, and marked by eminences and depres- 

 sions for the convolutions of the frontal lobes of the cerebrum. 



The border of the horizontal portion is thin, serrated, and articulates with 

 the lesser wing of the sphenoid. 



Structure. The frontal portion and external angular processes consist of diploic tissue sur- 

 rounded by compact bone. In the frontal sinus region the cancellous tissue is wanting. The 

 horizontal portion is thin, translucent, and composed entirely of compact tissue. 



Development (Fig. 46). The frontal bone is 

 formed in membrane, being developed from two 

 primary centres, one for each lateral half, which 

 make their appearance about the seventh or 

 eighth week, above the orbital arches. From 

 this point ossification extends, in a radiating 

 manner, upward into the forehead and backward 

 over the orbit. The nasal spine is developed 

 from two secondary centres, while additional cen- 

 tres appear in the regions of the internal and 

 external angular processes. Sometimes a centre 

 appears on either side at the lower end of the 

 coronal suture. This latter centre sometimes re- 

 mains ununited, and is known as the pterion 

 ossicle, or it may join with the parietal, sphenoid, 

 or temporal bone. At birth the bone consists 

 of two pieces, which afterward become united, 



along the median line, by a suture (metopic) which runs from the vertex to the root of the nose. 

 This suture usually becomes obliterated within a few years after birth, but it occasionally remains 

 throughout life. 



Articulations. With twelve bones two parietal, the sphenoid, the ethmoid, two nasal, two 

 maxillae, two lacrimal, and two malar. 



Attachment of Muscles. To three pairs the Corrugator supercilii, Orbicularis palpe- 

 brarum, and Temporal, on each side. 



FIG. 46. Frontal bone at birth. Developed from 

 two lateral halves. 



The Temporal Bone (Os Temporale). 



The temporal bone consists of three parts (a) the squamous, (6) the petro- 

 mastoid, and (c) the tympanic portions which, though separate in early life, 

 become united in the adult. The three parts meet and form a part of the 

 outer wall and a part of the base of the skull, and of the external auditory 

 meatus. 



The Squamous Portion (squamous temporalis), The squamous portion, the 

 anterior and upper part of the bone, is scale-like in form, and is thin and 

 translucent (Fig. 47). Its external surface is smooth, convex, and grooved at 

 its back part for the deep temporal arteries; it affords attachment to the Tem- 

 poral muscle and forms part of the temporal fossa. At its back part may be 

 seen a curved ridge (part of the temporal ridge}, which serves for the attachment 

 of the temporal fascia, limits the origin of the Temporal muscle, and marks 

 the boundary between the squamous and mastoid portions of the bone. Pro- 

 jecting from the lower part of the squamous portion is a long, arched process 

 of bone, the zygoma, or zygomatic process. This process is at first directed out- 

 ward, its two surfaces looking upward and downward; it then appears as if 

 twisted upon itself, and runs forward, its surfaces now looking inward and out- 

 ward. The superior border of the process is long, thin, and sharp, and serves 

 for the attachment of the temporal fascia. The inferior, short, thick, and arched, 

 has attached to it some fibres of the Masseter muscle. Its outer surface is convex 

 and subcutaneous; its inner is concave, and also affords attachment to the Masse- 

 ter. The extremity, broad and deeply serrated, articulates with the malar bone. 



