Skull and Hyoid 253 



present, although they have certainly existed where the bone is found divided,* as 

 is sometimes the case : possibly, however, a centre for the lower margin may be a 

 normal occurrence. 



The growing bone comes quickly into contact with the maxillary and temporal 

 bones, but it does not reach the frontal until later. At birth the temporal surface 

 has the appearance of a plate of bone applied to the remainder. 



THE ORBIT. 



Each orbit is a pyramidal-shaped space above the upper jaw. The upper surface 

 of the maxilla makes the greater part of the floor, but the malar comes into the outer 

 par, , the hamular process of the lachrymal may be in the floor internally, and the orbital 

 process of the palate is visible in the extreme back part of the floor at its junction with 

 the inner wall (see Fig. 185). 



The floor is separated from the outer or posterior wall by the spheno-maxillary 

 fissure, except at its extreme outer end, where the malar is continued from the floor 

 to the outer wall. The remaining and greater part of the outer wall is made by the 

 orbital surface of the great wing. The outer wall is separated from the roof by the 

 sphenoidal fissure in its back part, but outside this the great wing and the malar articu- 

 late with the frontal. 



The roof is almost entirely formed by the orbital plate of the frontal, but the small 

 wing of the sphenoid constitutes its most posterior portion. The inner wall is formed 

 by the body and turbinate of the sphenoid, in front of this the ethmoid, and in front of 

 this the lachrymal and the nasal process of the maxilla ; it has the anterior and posterior 

 ethmoidal canals between it and the roof, above the os planum, and the nasal duct 

 opens between it and the floor. 



The axis of the orbit is directed backwards and inwards and slightly upwards, 

 from the centre of the anterior opening to the optic foramen : the optic nerve lies 

 practically in the line of the axis. 



The margins of the anterior opening are made by several bones : above, the frontal, 

 internally the nasal process of the maxilla, the maxilla and malar below, and the malar 

 externally. Very rarely the hamular process of the lachrymal may be seen to reach 

 the margin of the orbit. The margins are sharp except at the inner and upper angle, 

 internal to the supraorbital foramen. 



Owing to the obliquity of the axis the inner walls of the orbits are in nearly parallel 

 planes, whereas the outer walls are in planes practically at right angles to each other. 

 The depth of the fossa is about 2 inches. 



The lachrymal sac lies in the lachrymal groove near the margin of the opening ; 

 the Tensor tarsi arises from the crest behind the groove and passes forward to the 

 outer side of the sac to join the Orbicularis palpebrarum, and the tendo oculi arises from 

 the nasal process in front of the groove and passes out across the middle of the sac. 



LOWER JAW OR MANDIBLE. 



The only bone in the skull (with the exception of the tympanic ossicles) that is 

 capable of separate movement : it carries the teeth opposing those of the upper jaw, 

 gives insertion to the muscles of mastication, and origin to muscles of the tongue and 

 floor of mouth and some muscles of expression. 



It consists of two halves which are strongly joined in the middle line at the sym- 



* This is rare, but may occur through a suture which divides the bone horizontally into an upper 

 larger) and lower (smaller) part. 



