330 THE ARTICULATIONS 



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-maxillary articulation is quite superficial, situated below the 

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

 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 either vertically or from side to side. 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 movement 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 also follow the exanthemata, and in adults occurs as the 

 result of some constitutional conditions, as rheumatism or gout. The temporo-maxillary joint is 

 also frequently 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 oppo- 

 site 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. 



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

 OF THE VERTEBRAE (Fig. 235). 



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, connected together by the following ligaments : 



Anterior Costo-central or Stellate. 

 Capsular. Interarticular. 



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

 the head of each rib with the sides of the bodies of two vertebrae and the inter- 

 vertebral disk between them. It consists of three flat bundles of ligamentous 

 fibres, which radiate from the anterior part of the head of the rib. The superior 

 fasciculus passes upward to be connected with the body of the vertebra above ; 

 the inferior one descends to the body of the vertebra below ; and the middle one, 

 the smallest and least distinct, passes horizontally inward, to be attached to the 

 intervertebral substance. 



Relations. In front, with the thoracic ganglia of the sympathetic, the pleura, 

 and, on the right side, with the vena azygos major; behind, with the interarticular 

 ligament and synovial membranes. 



In the first rib, which articulates with a single vertebra only, this ligament 

 does not present a distinct division into three fasciculi ; its fibres, however, radiate, 

 and are attached to the body of the last cervical vertebra, as well as to the body of 

 the vertebra with which the rib articulates. In the tenth, eleventh, and twelfth 

 ribs also, which likewise articulate with a single vertebra, the division does not 



