138 



OSTEOLOGY 



and posterior ethmoidal canals by the ethmoid, and open on the medial wall of the 

 orbit. The anterior canal transmits the nasociliary nerve and anterior ethmoidal 

 vessels, the posterior, the posterior ethmoidal nerve and vessels. In front of the 

 ethmoidal notch, on either side of the frontal spine, are the openings of the frontal 

 air sinuses. These are two irregular cavities, which extend backward, upward, 

 and lateralward for a variable distance between the two tables of the skull; they 

 are separated from one another by a thin bony septum, which often deviates to 

 one or other side, with the result that the sinuses are rarely symmetrical. Absent 

 at birth, they are usually fairly well-developed between the seventh and eighth 

 years, but only reach their full size after puberty. They vary in size in different 

 persons, and are larger in men than in women. 1 They are lined by mucous mem- 

 brane, and each communicates with the corresponding nasal cavity by means of a 

 passage called the frontonasal duct. 



Borders. The border of the squama is thick, strongly serrated, bevelled at the 

 expense of the inner table above, w r here it rests upon the parietal bones, and at 

 the expense of the outer table on either side, where it receives the lateral pressure 

 of those bones; this border is continued below into a triangular, rough surface, 

 which articulates with the great wing of the sphenoid. The posterior borders of 

 the orbital plates are thin and serrated, and articulate with the small wings of the 

 sphenoid. 



Structure. The squama and the zygomatic processes are very thick, consisting of diploic 

 tissue contained between two compact laminae; the diploic tissue is absent in the regions occupied 

 by the frontal air sinuses. The orbital portion is thin, translucent, and composed entirely of 

 compact bone; hence the facility with which instruments can penetrate the cranium through 

 this part of the orbit; when the frontal sinuses are exceptionally large they may extend backward 

 for a considerable distance into the orbital portion, which in such cases also consists of only two 

 tables. 



Ossification (Fig. 136). The frontal bone is ossified in membrane from two primary 

 centers, one for each half, which appear toward the end of the second month of fetal life, one 

 above each supraorbital margin. From each of these centers ossification extends upward to form 

 the corresponding half of the squama, and backward to form the orbital plate. The spine is 



ossified from a pair of secondary centers, 



on either side of the middle line; similar 



centers appear in the nasal part and zvgo- 

 matic processes. At birth the bone consists 

 of two pieces, separated by the frontal 

 suture, which is usually obliterated, except 

 at its lower part, by the eighth year, but 

 occasionally persists throughout life. It is 

 generally maintained that the development 

 of the frontal sinuses begins at the end of 

 the first or beginning of the second year, 

 but Onodi's researches indicate that de- 

 velopment begins at birth. The sinuses 

 are of considerable size by the seventh or 

 eighth year, but do not attain their full 

 proportions until after puberty. 



Articulations. The frontal articulates 

 with twelve bones: the sphenoid, the eth- 

 moid, the two parietals, the two nasals, the 

 two maxillae, the two lacrimals, and the 

 two zygomatics. 



Nasal part 



Zygomatic process 



Spine 

 Fia. 136. Frontal bone at birth. 



The Temporal Bone (Os Temporale). 



The temporal bones are situated at the sides and base of the skull. Each consists 

 of five parts, viz., the squama, the petrous, mastoid, and tympanic parts, and the 

 styloid process. 



1 Aldren Turner (The Accessory Sinuses of the Nose, 1901) gives the following measurements for a sinus of average 

 size: height, \ l /i inches; breadth, 1 inch; depth from before backward, 1 inch. 



