326 



THE ARTICULATIONS. 



nect them with the articular processes of the atlas ; they consist of thin and loose 

 capsules, which enclose the synovial membrane of the articulation. 



Synovial Membranes. There are two synovial membranes in this articulation, 

 one lining the inner surface of each of the capsular ligaments. These occasionally 

 communicate with that between the posterior surface of the odontoid process and 

 the transverse ligament. 



Actions. The movements permitted in this joint are flexion and extension, 

 which give rise to the ordinary forward and baclovard nodding of the head, besides 



FIG. 230. Articulation between odontoid process and atlas. 



slight lateral motion to one or the other side. When either of these actions is 

 carried beyond a slight extent, the whole of the cervical portion of the spine assists 

 in its production. Flexion is mainly produced by the action of the Rectus capitis 

 anticus major et minor and the Sterno-mastoid- muscles ; extension by the Rectus 

 capitis posticus major et minor, the Superior oblique, the Complexus, Splenius, 

 and upper fibres of the Trapezius. The Recti laterales are concerned in the lat- 

 eral movement, assisted by the Trapezius, Splenius, Complexus, Sterno-mastoid, 

 and the Recti laterales of the same side, all acting together. According to Cru- 

 veilhier, there is a slight motion of rotation in this joint. 



IV. Articulation of the Axis with the Occipital Bone. 



Occipito-axial. Three Odontoid. 



To expose these ligaments the spinal canal should be laid open by removing 

 the posterior arch of the atlas, the laminge and spinous process of the axis, and 

 the portion of the occipital bone behind the foramen magnum, as seen in Fig. 231. 



The Occipito-axial Ligament (apparatus ligamentosus colli) is situated within 

 the spinal canal. It is a broad and strong ligamentous band, which covers the 

 odontoid process and its ligaments, and appears to be a prolongation upward of 

 the posterior common ligament of the spine. It is attached, below, to the posterior 

 surface of the body of the axis, and, becoming expanded as it ascends, is inserted 

 into the basilar groove of the occipital bone, in front of the foramen magnum, where 

 it becomes blended with the dura mater of the skull. 



Relations. By its anterior surface with the transverse ligament, by its posterior 

 surface with the dura mater. 



The Lateral Odontoid or Check Ligaments (alar ligaments) are strong, fibrous 

 cords, which arise one on either side of the upper part of the odontoid process, 

 and, passing obliquely upward and outward, are inserted into the rough 

 depressions on the inner side of the condyles of the occipital bone. In the 

 triangular interval between these ligaments another strong fibrous cord (liga- 

 mentum suspensorium or middle odontoid ligament) may be seen, which passes 

 almost perpendicularly from the apex of the odontoid process to the anterior 

 margin of the foramen, being intimately blended with the deep portion of the 



