1346 CLINICAL AND TOPOGRAPHICAL ANATOMY 



the mandible — unilateral, near the mental foramen — the larger anterior fragment 

 will be pulled by the depressors downward and medially, the smaller posterior one 

 upward and usually lateral to the other fragment. 



Maxilla. — The boundaries of the maxillary sinus (antrum) are of much im- 

 portance. The base of this irregularly pyramidal cavity corresponds to the 

 middle and inferior meatuses on the lateral wall of the nose; toward the upper 

 and back part is the opening into the middle meatus. The apex runs laterally 

 toward the zygomatic process. The roof is formed by the orbital plate with the 

 infraorbital nerve and vessels anteriorly; the floor by the junction of the alveolar 

 arch, carrying the first molars (and often the bicuspids), with the hard palate. 

 It may be pierced by the roots of the second bicuspid or first and second molar 

 teeth. Anteriorly, the antrum is bounded by the canine fossa; posteriorly it is 

 in relation vdih the zygomatic fossa. The cavity, present at birth, increases 

 gradually up to the twelfth year. 



The chief paths of infection are through the teeth (especially the first and second molar), 

 the nose, and frontal sinus. The obstinacy of inflammation here is explained by the site of the 

 opening, high up on the medial wall, and thus inadequate drainage, by the imperfectly multi- 

 locular cavity of the interior and its rigid walls. The chief sites for opening the antrum are — (a) 

 through the sockets of the first or second molars; (b) through the canine fossa, after the reflec- 

 tion of mucous membrane has been detached, midway between the roots of the teeth and the 

 infraorbital foramen (this path gives more room); (c) through the inferior meatus of the nose. 



THE ORBIT AND EYE 



The bony orbit is a pyramidal fossa with its base at the orbital margin and its 

 apex at the optic foramen. The medial walls of the two orbits are approximately 

 parallel, but the lateral walls diverge as they are traced forward and lie at right 

 angles to each other. The thin floor which is formed mainly by the maxilla and 

 corresponds to the roof of the maxillary sinus, is readily destroyed by growths 

 extending up from the sinus and in the process pressure on the infraorbital 

 nerve is apt to cause pain referred to the cheek. The roof formed by the 

 orbital plate of the frontal bone is also thin, and foreign bodies thrust into the 

 orbit may perforate it and enter the frontal lobe of the cerebrum. The medial 

 wall is chiefly constituted by the lacrimal and lamina papyracea of the ethmoid, 

 both very thin bones. This wall is readily destroj^ed by malignant growths of the 

 nose. 



Injuries of the medial wall such as may be associated with fractures of the nose bring the 

 ethmoidal air cells into communication with the cellular tissue of the orbit. The latter may thus 

 be distended with air on attempting to blow the nose. 



The lateral wall is formed in its anterior third by the zygomatic bone, which 

 separates the orbit from the zygomatic fossa. The posterior two-thirds formed 

 by the sphenoid bone separate the orbit from the temporal lobe of the brain in 

 the middle cranial fossa. The orbit communicates with the cranium by the 

 optic foramen, which transmits the optic nerve and ophthalmic artery and the 

 superior orbital fissure through which pass all the other vessels and nerves of the 

 orbit. 



In cases of fracture of the base of the skull involving the anterior clinoid process, a traumatic 

 communication (arterio-venous aneurysm) may be formed between the internal carotid artery 

 and cavernous sinus, behind the :ij)ox of the orbit, giving rise to pulsating exophthalmos. 



The orbital margin is larger in the transverse than in the vertical direction, 

 and consequently there is more space on either side than above and below be- 

 tween it and the eyeball which is nearly spherical. The eyeball lies nearer to the 

 medial than to the lateral margin and hence foreign bodies more commonly 

 penetrate the orbit to the lateral side of the eye. 



Behind the fascia bulbi, the eyeball rests on a mass of soft loose orbital fat in 

 which fon'ign bodies may be hidden for a considerable time. 



The structure of the eyelids. — The different layers are of much practical 

 importance. (1) The skin is delicate and fatless, and contains pigment, the 

 object of this being to protect the eye from bright light. It helps to explain the 

 'dark circles' of later life. (2) Areolar tissue. ()wing to its looseness and 

 delicacy, this is very liable to infiltration, us in oedema and erysipelas. (3) 



