48 OSTEOLOGY. 



in shape, and called the os triquetrum, or Wormian bone ; the 

 ossific tentorium springs from this bone when present. The 

 parietal bone articulates with its fellow, the occipital, frontal, 

 squamosal, petrosal, and sphenoid — i.e., with six bones, or with 

 seven Avhen the Wormian bone is present. 



FRONTAL BONE. 

 (Figs. 10. IV. ; 12. and 13. 3.) 



This bone is situated at the antero-superior part of the cranium, 

 and the pair constitute the broad, flat part called the forehead ; 

 their union in the centre of the forehead forms the frontal suture. 

 Each frontal bone is irregular in shape, flat externally, smooth and 

 concave internally, and presents external and internal surfaces 

 and four borders. 



The external surface is somewhat prominent above and slightly 

 depressed below, and has a lateral process, the external orbital, 

 which arches downwards and slightly backwards, and bounds part 

 of the orbital fossa ; it articulates with the zygomatic process of 

 the squamosal bone, and is also termed the frontal arch. The 

 foramen piercing the superior part of this process is the supra- 

 orbital, which transmits the artery, vein, and nerve of the same 

 name. The thin plate which extends downwards and back- 

 wards from the external lateral border of the bone is the internal 

 orbital pkite or process, divided by a large triangular opening- 

 closed, in the articulated state, by the orbital portion of the 

 sphenoid ; and the small notch in front of the large one forms, 

 with the sphenoid bone, the internal orbital foramen. The pit 

 or hollow above the larger notch lodges the lachrymal gland ; 

 and the small depression at the base of the external orbital 

 process is for the attachment of the greater oblique muscle of the 

 eye. The slightly-depressed part behind the orbital process assists 

 in forming the temporal fossa. 



The internal surface is irregularly concave, and divided into 

 two unequal parts by a bony septum, the cranial plate, which 

 meets the cribriform plate of the ethmoid bone. The posterior 

 division is smooth, and marked by ridges, which correspond with 

 the convolutions of the brain, the anterior lobe of which it covers ; 

 a furrow for the anterior meningeal artery runs upwards from 

 the internal orbital foramen, and close to the ethmoid bone. 

 The continuation of the longitudinal groove of the parietal bone 



