234 THE ARTICULATIONS 



ment takes place principally in the upper compartment of the joint: the fibro- 

 cartilage, carrying with it the condyle, glides forward on the glenoid fossa. This 

 is because this movement is mainly effected by the External pterygoid muscles, 

 which are inserted into both condyle and interarticular fibro-cartilage. The grind- 

 ing or chewing movement is produced by the alternate movement of one condyle, 

 with its fibro-cartilage, forward and backward, while the other condyle moves 

 simultaneously in the opposite direction ; at the same time the condyle undergoes a 

 vertical rotation on its own axis on the fibro-cartilage in the lower compartment. 

 One condyle advances and rotates, the other condyle recedes and rotates, in alter- 

 nate succession. 



The lower jaw is depressed by its own weight, assisted by the Platysma, the 

 Digastric, the Mylo-hyoid, and the Genio-hyoid. It is elevated by the anterior 

 part of the Temporal, Masseter, and Internal pterygoid. It is drawn forward by 

 the simultaneous action of the External pterygoid and the superficial fibres of the 

 Masseter; and it is drawn backward by the deep fibres of the Masseter and the 

 posterior fibres of the Temporal muscle. The grinding movement is caused by the 

 alternate action of the two External pterygoids. 



Surface Form. The temporo-mandibular articulation is quite superficial, situated below the 

 base of the zygoma, in front of the tragus and external auditory meatus, and behind the poste- 

 rior border of the upper part of the Masseter muscle. Its exact position can be at once ascer- 

 tained by feeling for the condyle of the jaw, the working of which can be distinctly felt in the 

 movements of the lower jaw in opening and shutting the mouth. When the mouth is opened 

 wide, the condyle advances out of the glenoid fossa on to the eminentia articularis, and a depres- 

 sion is felt in the situation of the joint. 



Surgical Anatomy. The lower jaw is dislocated only in one direction viz. forward. The 

 accident is caused by violence or muscular action. When the mouth is open, the condyle is 

 situated on the eminentia articularis, and any sudden violence, or even a sudden muscular spasm, 

 as during a convulsive yawn, may displace the condyle forward into the zygomatic fossa. The 

 displacement may be unilateral or bilateral, according as one or both of the condyles are dis- 

 placed. The latter of the two is the more common. 



Sir Astley Cooper described a condition which he termed "subluxation." It occurs 

 principally in delicate women, and is believed by some to be due to the relaxation of the liga- 

 ments, permitting too free niovement of the bone, and possibly some displacement of the fibro- 

 cartilage. Others have believed that it is due to gouty or rheumatic changes in the joint. In 

 close relation to the condyle of the jaw is the external auditory meatus and the tympanum ; any 

 force, therefore, applied to the bone is liable to be attended with damage to these parts, or 

 inflammation in the joint may extend to the ear, or on the other hand inflammation of the middle 

 ear may involve the articulation and cause its destruction, thus leading to ankylosis of the joint. 

 In children, arthritis of this joint may follow the exanthemata, and in adults it occurs as the 

 result of some constitutional conditions, as rheumatism or gout. The temporo-mandibular 

 joint is also occasionally the seat of osteo-arthritis, leading to great suffering during efforts of 

 mastication. A peculiar affection sometimes attacks the neck and condyle of the lower jaw, 

 consisting in hypertrophy and elongation of these parts and consequent protrusion of the chin 

 to the opposite side. 



VI. Articulations of the Ribs with the Vertebrae. 



The articulations of the ribs with the vertebral column may be divided into 

 two sets : 1. Those which connect the heads of the ribs with the bodies of the 

 vertebrae, costo-centraL 2. Those which connect the necks and tubercles of the 

 ribs with the transverse processes, costo-transverse. 



I. ARTICULATIONS BETWEEN THE HEADS OF THE RIBS AND THE BODIES 

 OF THE VERTEBRAE (Fig. 160). 



These constitute a series of arthrodial joints, formed by the articulation of the 

 heads of the ribs with the cavities on the contiguous margins of the bodies of the 

 dorsal vertebrae and the intervertebral substance between them, except in the case 

 of the first, tenth, eleventh, and twelfth ribs, where the cavity is formed by a single 

 vertebra. The bones are connected by the following ligaments : 



Anterior Costo-vertebral or Stellate. 

 Capsular. Interarticular. 



The Anterior Costo-vertebral or Stellate Ligament connects the anterior part of 



