ARTICULATIONS OF THE HEAD. 



187 



the great oblique muscle of the head ; its internal responds, in its inferior half, 

 to the articular synovial membrane, and its superior moiety to the spinal dura 

 mater. It is analogous to the capsules in other regions. 

 (Leyh describes this Ugament as the interannular). Fig. 124. 



Synovial membrane. — This lines the odontoid ligament 

 and atlo-axoid ligaments, and the articular portion of the 

 peripheral capsule. 



Movements. — Rotation, the only movement possible in 

 the atlo-axoid articulation, is effected in the following 

 manner : the axis remains fixed, and the first vertebra, 

 drawn to one side chiefly by the great oblique muscle, 

 rotates on the odontoid pivot, carrying the head with it. 



The rotation movements of the head have, therefore, 

 this diarthrosis for a centre, and not the atloido-occipital 

 articulation. 



In the Dog and Cat, the odontoid ligament is replaced by three 

 particular ligaments : 1. Two lateral cords, rising in common from the 

 summit of the odontoid process, and inserted, each on its own side, 

 within the condyles of tlie occipital bone. 2 A transverse ligament, 

 passing over the odontoid process, which it maintains in its place 

 against the itiferior arch of the atlas, and is attached by its extremities 

 to the superior face of the latter. A small synovial capsule facili- 

 tates the gliding of the odontoid process beneath this ligament. The 

 articular synovial membrane always communicates with that of the 

 occipito-atloid articulation. 



In the Pig, the disposition is nearly the same as in the Caruivora. 



2. Occipito-atloid Articulation (Fig. 124). 



(^Preparation. — Dissect away all the soft parts that pass from the 

 neck to the head and cover the articulation, and more particularly 

 the flexor, the recti, and the small oblique muscles of the head. To 

 expose the synovial membranes, open the sides of the capsular 

 ligament.) 



This is a condyloid articulation. 



Articular surfaces. — In the atlas, the two cavities which 

 replace the anterior articular processes and the heads of the 

 other vertebrfe ; in the occipital bone, the two condyles 

 flanking the sides of the occipital foramen. 



Mode of union. — A single capsular ligament envelops 

 the entire articulation ; it is attached by its anterior border 

 to the margin of the occipital condyles, and by its posterior 

 to the anterior contour of the atlas. Thin and slightly 

 elastic in its inferior half, this ligament presents, supe- 

 riorly, four reinforcing fasciculi : two middle, which inter- 

 cross in X — whence the name "cruciform," sometimes 

 given to this ligament (Fig. 124, 1, 1) ; and two lateral, 

 which pass from the sides of the atlas to the base of 

 the styloid processes (Fig. 124, 2, 2). It is lined by 

 the synovial membranes, and is enveloped externally by a 



large number of muscles, which protect the articulation and greatly strengthen 

 it everywhere. Among these may be particularly noticed the recti muscles of 



ATLO-OXOID AND OCCIPI- 

 TO-ATLOID ARTICULA- 

 TIONS (the upper arch 

 of the atlas has been 

 removed to show the 

 odontoid ligament). 



1, 1, Middle accessory fas- 

 ciculi ; 2, 2, lateral 

 fasciculi of the capsular 

 ligament of the occipi- 

 to-atloid articulation ; 



3, odontoid ligament ; 



4, interspinous liga- 

 ment uniting the second 

 and third vertebrae of 

 the neck ; 5, fibrous 

 capsule uniting the 

 articular processes of 

 these vertebrae. A, 

 Anterior internal fora- 

 men of the atlas con- 

 verted into a groove by 

 the section of the bone; 

 B, B, vertebral foramina 

 of the atlas ; C, C, fora- 

 mina replacing the an- 

 terior notches of the 

 axis. 



