140 ARTICULATIONS OF THE TEUXK AXD HEAD. 



The I/{/ammium susjirnsorium dentls or middle odontoid ligament 



consists of fibres which pass directly upwards from the summit of the 

 odontoid process to the margin of the foramen magnum. 



The occipito-axial ligament sometimes called apparatus ligamen- 

 tosus colli, is the continuation upwards of the posterior common liga- 

 ment, and by its breadth covers the cruciform and odontoid ligaments. 

 It is attached above in the basilar groove, and below to the body of 

 the axis. 



The anterior occipitc-atlantal ligament extends from the ante- 

 rior border of the occi])ital foramen, between the condyles, to the anterior 

 arch of the atlas. It is thin, broad, and membranous ; but in the 

 median line it is strengthened by an accessory ligament, thick and 

 round, placed in front of it, which is sometimes described as the com- 

 mencement of the anterior common ligament. 



The anterior atlanto-axial ligament, likewise thin and mem- 

 branous, except in the middle, where it is considerably thickened, 

 extends from the anterior arch of the atlas to the body of the axis. 



The posterior occipitc-atlantal ligament, thin and membranous, 

 is attached superiorly to all that part of the margin of the occipital 

 foramen which is behind the condyles, and inferiorly to the adjacent 

 "border of the arch of the atlas. It is partly blended with the dura 

 mater. 



The posterior atlanto-axial ligament, similar to the preceding, 

 connects the neural arch of the atlas with that of the axis, in the 

 absence of ligamentum subflavum. 



Movements. — The atlanto-axial articulation is so constracted that the head, 

 together with the atlas, is rotated on the axis : the odontoid process serving as a 

 pivot. The rotation is limited by the check ligaments. The occipito-atlantal 

 articulation takes no part in rotation, but allows the head to be freely raised or 

 depressed upon the vertebral column. "Wlien the atlas is placed symmetrically 

 over the axis, it is seen that the opposing articular surfaces, instead of fitting 

 one to the other, come very slightly into contact, the surface of the axis being 

 inclined too little outwards, and presenting an antero-posterior convexity, to 

 which there is no corresponding concavity presented by the atlas : but a slight 

 rotation brings the bones into a stable position, in which the anterior half of 

 one articular surface of the axis and the posterior half of the other, are laid 

 closely against the atlas. It wall also be found that a small amoimt of oblique 

 motion between the atlas and occipital bone is permitted, by which the anterior 

 half of one condyle and the posterior part of the other may be rested together on 

 the atlas, and that that is the position of greatest stability. This oblique posi- 

 tion is that into which the bones are brought when there is any lateral flexure 

 of the column, as is the case m the most natural and easy attitudes. 



ARTICULATIONS OF THE PaBS. 



The articulations of the ribs may be divided into three sets, costo- 

 central, costo-transverse, and costo-sternal. 



The cosTO-CENTRAL ARTICULATION Unites the head of the rib, in most 

 instances, with the bodies of two vertebrai by two distinct synovial 

 joints, supported by ligaments as follows. 



The anterior costo-central ligament, radialcd or sU'llate, consists 

 of three bundles, of which the middle one j^asses horizontally forwards 

 upon the corresponding intervertebral fibro-cartilage, the superior 

 ascends to the body of the vertebra above it, and the inferior descends 



