48 THE BONES. 



lachrymal, and temporal bones. It is described as having two faces, two 

 borders, a base, and a summit. 



Faces. The external face comprises two portions separated from each 

 other by a semicircular ledge that extends from the summit to the middle of 

 the anterior border of the bone, and concurs to form the outer margin of the 

 orbit. The anterior portion, smooth and concave, belongs to the orbital 

 cavity. The posterior, more extensive, is also smooth and slightly convex. 

 The internal face is excavated in its central part, which corresponds to the 

 maxillary sinus. On its margin it shows dentations and lamellae for articu- 

 lation with the supermaxillary bone. 



Borders. The anterior, thin and denticulated, is joined to the lachrymal 

 bone. The posterior, or masseteric border, is thicker, and constitutes a 

 roughened crest, the zygomatic ridge, which is continued above with the 

 posterior border of the process of the same name, and below with the 

 maxillary spine. 1 



Base and summit. The base, very thin, is united to the supermaxillary 

 bone. The summit, flattened from before to behind and bevelled on its 

 anterior face, joins the zygomatic process, and forms with it the jugal bridge 

 or zygomatic arch. 



Structure and development. This bone is rather spongy in its upper part, 

 and is developed from a single nucleus of ossification. 



6. Lachrymal Bone. 



A small, thin, and very light bone, bent on itself at a right angle, it is situ- 

 ated beneath the orbit, which it aids in forming, and is wedged between the 

 frontal, nasal, supermaxillary, and zygomatic bones. It is studied on its 

 external and internal faces and circumference. 



Faces. The external is divided into two regions, superior and inferior, 

 by a curved crest which forms part of the orbital margin, and is provided 

 with notches, which are variable in their form and number. The superior 

 region, named the orbital, because of its situation in the orbit, is slightly 

 concave and smooth. It presents, near the orbital margin, the orifice of the 

 lachrymal duct, which traverses the maxillary sinus and opens on the internal 

 face of the supermaxillary bone, where it is continued by a fissure ; behind 

 this is the lachrymal fossa. The inferior or facial region is slightly bulging, 

 and provided sometimes with a tubercle of insertion, the lachrymal tubercle. 

 The internal face is employed, for the whole of its extent, in the formation of 

 the walls of the maxillary and frontal sinuses ; it exhibits a cylindrical 

 prominence produced by the bony tube of the lachrymal duct. 



Circumference. This is very irregular and denticulated to respond to 

 the neighbouring bones. 



Structure and development. This bone is entirely compact, and is 

 developed from a single nucleus of ossification. 



7. Nasal Bones. 



Situated on the anterior aspect of the head, these bones articulate with 

 each other in the median line, and are fixed between the frontal, lachrymal, 

 and supermaxillary bones; they are triangular in shape, elongated from 

 above to below, flattened from before to behind, and offer for study two faces, 

 two borders, a base, and a summit. 



Faces. The external or anterior face, larger above than below, is convex 



1 The designation of zygomatic crest is often given to these three parts collectively. 



