246 HUMAN ANATOMY. 



of acute disease, the intra-articular cartilage being so arranged (page 214) that it 

 acts as an elastic buffer presenting one surface upon which the hinge-like, and 

 another upon which the sliding, movement of the jaw may take place. Suppurative 

 disease of the middle ear may extend to the joint (Barker). 



Rheumatoid arthritis is perhaps the most common disease of the joint, and may 

 be localized there in subjects otherwise predisposed by the frequent exposure of the 

 joint to cold and wet. 



The so-called "subluxation," sometimes, perhaps, depending upon relaxation 

 of the ligaments, is more probably in the majority of cases due to rheumatic or gouty 

 changes in the joint. 



Dislocation of the jaw (discussed in connection with the action of the associ- 

 ated muscles, page 493) occurs only when the mouth is widely open, as in yawning, 

 so that the condyle passes beyond its proper limits, over the summit of the ridge, 

 and is lodged in front. " When the mouth is widely opened the condyles, together 

 with the interarticular fibro-cartilage, glide forward. The fibro-cartilage extends 

 as far as the anterior edge of the eminentia articularis, which is coated with cartilage 

 to receive it. The condyle never reaches quite so far as the summit of that emi- 

 nence. All parts of the capsule save the anterior are rendered tense. The coronoid 

 process is much depressed. Now, if the external pterygoid muscle (the muscle 

 mainly answerable for the luxation) contract vigorously, the condyle is soon drawn 

 over the eminence into the zygomatic fossa, the interarticular cartilage remaining 

 behind. On reaching its new position it is immediately drawn up by the temporal, 

 internal pterygoid, and masseter muscles, and is thereby more or less fixed. A 

 specimen in the Mus6e Dupuytren shows that the fixity of the luxated jaw may 

 sometimes depend upon the catching of the apex of the coronoid process against 

 the malar bone' ' (Treves). 



Excision of the inferior maxilla, since it is concerned chiefly with the soft parts, 

 will be considered in connection with the Muscles (page 493). 



Landmarks. — The supra-orbital ridges mark the boundary between the face 

 and the cranium. The supra-orbital notch can be felt at the junction of the inner 

 and middle thirds of the supra-orbital margin. A line from that point to the 

 interval between the two bicuspid teeth in both jaws crosses the infra-orbital and 

 the mental foramina (Holden). 



The attachment of the nasal cartilages to the superior maxillae and to the nasal 

 bones can easily be felt. The connective tissue between the skin and the cartilages 

 is very scanty. This is a source of difficulty in some of the plastic operations on the 

 ' nose, and is also a cause of the severe pain felt in cellulitis and in furuncles of that 

 region. The great vascularity of the part and the fact that ' ' the edge of the nostril 

 is a free border and the circulation therefore is terminal" (Treves) favor congestion 

 and engorgement, while the close connection of the skin and cartilage resists the 

 swelling ; hence the nerve-pressure and the excessive pain. 



The malar prominence, the concavity of the superior maxilla representing the 

 anterior wall of the antrum, its malar process, corresponding to the apex of that 

 cavity, the incisor fossa, and the canine fossa can easily be recognized either through 

 the cheek or, more readily, through the gums with a finger in the mouth. 



The zygoma can be both seen and felt, the lower border more distinctly than 

 the. upper on account of the attachment to the latter of the dense temporal fascia. 

 Wasting diseases cause an apparent increase in the prominence of the zygoma. 



The condyle of the inferior maxilla can be outlined and its motions observed 

 (Fig. 246) just in advance of the ear. 



A line drawn from the angle to the condyle indicates the posterior border of 

 the ramus. In making incisions in this region for inflammatory or suppurative 

 conditions this line is to be remembered. Posterior to it important blood-vessels 

 may be injured ; anterior to it deep punctures may be made with safety, the only 

 structure of cofisequence endangered being branches of the facial nerve. 



From the angle of the jaw forward the outline of the inferior maxilla can be 

 seen and felt both externally and within the mouth. The alignment of the teeth is 

 usually disturbed in fracture, and is often the most easily recognized symptom. 

 With a finger between the cheek and the teeth, the anterior border of the coronoid 



