158 



THE ARTICULATIONS OF THE TRUNK AND HEAD. 



to the adjacent border of the arch of the atlas : it is closely united to the dura 

 mater. A small band on each side forms an arch between the back of the vertebral 

 groove and the posterior end of the superior articular process of the atlas, thus 

 completing a foramen through which pass the vertebral artery and the suboccipital 

 nerve. This arch is not unfrequently ossified (p. 8). 



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

 neural arch of the atlas with that of the axis, in the place of ligamenta subfiava. 

 It is perforated on each side by the second cervical nerve. 



Movements. The atlanto-axial articulation is so constructed 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 : its extent is about 30 to either side. The occipito- 

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

 depressed upon the vertebral column. When 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 presenting an an tero -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 will also be 

 found that a small amount of oblique motion between the atlas and occipital bone is per- 

 mitted, 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 

 position is that into which the bones are brought when there is any lateral flexion of the 

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



THE ARTICULATIONS OF THE THORAX. 



The articulations of the ribs with the vertebrae are divided into costo-centml and 

 costo-transverse. Anteriorly the costal cartilages are connected to the sternum by 



Fig. 184. THE STH, CTH, TTH, STH, AND 9iH DORSAL 



VERTEBR.E, WITH PARTS OF THE 6TH, ?TH, AND 

 1 8TH RIBS, FROM THE RIGHT SIDE AND FRONT. 



(Allen Thomson.) ^ 



The 9th rib has been removed to show the articular 

 surfaces of the vertebras corresponding to it ; 3 and 4, 

 the heads of the 6th and 7th ribs, from which the 

 stellate ligaments are seen spreading over the two adja- 

 cent vertebral bodies and intervertebral substance ; 5, 

 the head of the 8th rib, from which the stellate liga- 

 ment has been removed, so as to expose the upper 

 and lower synovial cavities, and between them the 

 interarticular ligament ; 6, lower, and 6', upper facet 

 of the costo-central articulation ; 7, posterior costo- 

 transverse ligament ; 7', the costo-transverse .synovial 

 cavity ; 7", the costo-transverse articular facet ; 8, 

 anterior superior costo-transverse ligament ; 9, superior 

 articular process of the 5th vertebra; 9', inferior of the 9th. 



the chondro-sternal articulations, and with 

 one another by the interchondral articulations. 

 The three portions of the breast-bone are 

 united by the sternal articulations. 



The COSTO-CENTRAL ARTICULATION unites 



the head of the rib, in most instances, with 

 the bodies of two vertebrae by two distinct 



synovial joints, supported by ligaments as follows. 



The anterior costo-central or stellate ligament consists of fibres which 



radiate from the head of the rib to the body of its proper vertebra, to the intervertebral 



disc, and to the body of the vertebra above. Short fibres surround the remaining 



portion of the articulation and complete a capsule to the joint. 



