OF THE RIBS WITH THE VERTEBRAE. 



275 



muscle. It is thicker at its circumference, especially behind, than at its centre, 

 where it is sometimes perforated. The fibres of which it is composed have a 

 concentric arrangement, more apparent at the circumference than at the centre. 

 Its surfaces are smooth, and divide 



the joint into two cavities, each of Fig. 190. Vertical Section of Temporo-maxillary 

 which is furnished with a separate Articulation, 



synovial membrane. When the 

 fibre-cartilage is perforated, the sy- 

 novial membranes are continuous 

 with one another. 



The Synovial Membranes, two in 

 number, are placed one above, and 

 the other below the fibro-cartilage. 

 The upper one, the larger and looser 

 of the two, is continued from the 

 margin of the cartilage covering the 

 glenoid cavity and eminentia articu- 

 laris, over the upper surface of the 

 fibro-cartilage. The lower one is 

 interposed between the under sur- 

 face of the fibro-cartilage and the condyle of the jaw, being prolonged down- 

 wards a little further behind than in front. 



The nerves of this joint are derived from the auriculo-temporal and masse- 

 teric branches of the inferior maxillary. 



Actions. The movements permitted in this articulation are very extensive. 

 Thus, the jaw may be depressed or elevated, or it may be carried forwards or 

 backwards, or from side to side. It is by the alternation of these movements 

 performed in succession, that a kind of rotary movement of the lower jaw upon 

 the upper takes place, which materially assists in the mastication of the food. 



If the movement of depression is carried only to a slight extent, the condyles 

 remain in the glenoid cavities, their anterior part descending only slightly ; but 

 if depression is considerable, the condyles glide from the glenoid fossas on to 

 the articular eminences, carrying with them the interarticular fibro-cartilages. 

 "When this movement is carried to too great an extent, as, for instance, during a 

 convulsive yawn, dislocation of the condyle into the zygomatic fossa may occur; 

 the interarticular cartilage being carried forwards, and the capsular ligament 

 raptured. When the jaw is elevated, after forced depression, the condyles and 

 fibro-cartilages are carried backwards into their original position. When the 

 jaw is carried horizontally forwards and backwards, or from side to side, a 

 horizontal gliding movement of the fibro-cartilages and condyles upon the 

 glenoid cavities takes place in the corresponding direction. 



VI. ARTICULATION OF THE RIBS WITH THE VERTEBRAE. 



The articulation 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 

 vertebras; 2. Those which connect the neck and tubercle of the ribs with the 

 transverse processes. 



1. ARTICULATION BETWEEN THE HEADS OF THE EIBS AND THE BODIES OF THE 



VERTEBRA. (Fig. 191.) 



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

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

 of the dorsal vertebra?, connected together by the following ligaments : 



Anterior Costo- vertebral or Stellate. 



Capsular. 



Interarticular. 



