136 THE ARTICULATIONS. 



the superior odontoid ligament and the synovial membrane, one half the atlas and axis 

 must be separated by sawing longitudinally through them from one side to the other.) 



This may be considered as the type of the trochoides. 



Articular surfaces. To form this articulation, the axis offers its odontoid 

 pivot and the undulated diarthrodial facets at its base. The atlas opposes 

 to the pivot the concave semicylindrical surface hollowed on the superior 

 face of its body ; and for the lateral undulated facets it has analogous facets 

 which are cut on the transverse processes, on each side of the vertebral canal. 



Mode of union. 1. An odontoid, or odonto-atloid ligament; 2. An 

 inferior atlo-axoid ligament ; 3. A superior ditto ; 4. A fibrous capsule. 



a. Odontoid ligament (Fig. 81, 3). Continued to the common superior 

 vertebral ligament, very short and strong, flattened from above to below, 

 and triangular in shape, the odontoid ligament is composed of glistening 

 white fibres, fixed behind in the superior channel of the odontoid process, . 

 and inserted in front on the transverse ridge which separates the superior 

 face from the inferior arch of the atlas, as well as on the imprints situated 

 in front of this ridge. This ligament is covered, on its lower face, by the 

 synovial membrane of the articulation; and by its upper surface is in 

 contact with the spinal dura mater. It sends some bands within the 

 condyles of the occipital bone. 



b. Inferior atlo-axoid ligament. This is a wide, thin, and nacrous- 

 looking band, extending from the inferior face of the axis to the inferior 

 tubercle of the atlas, and covered by the long muscle of the neck ; it is 

 united to the synovial membrane by its deep face, and confounded on its 

 borders with the fibrous capsule to be immediately described. 



c. Superior atlo-axoid ligament. This exactly represents the inter- 

 spinous ligaments of the other cervical articulations. Yellow, elastic, and 

 formed like the two lateral bands, it is continuous, laterally, with the 

 capsular ligament. 



d. Capsular ligament. This, it may be said, is only the interlamellar 

 ligament proper to the atlo-axoid articulation. It commences from the 

 sides of the preceding ligament, and becomes united to the inferior atlo- 

 axoid one, after contracting adhesions with the borders of the odontoid 

 ligament. In this way it encloses the articulation and the spinal canal. 

 Before and behind, it is attached to the anterior or posterior margin of 

 the bones it unites. Its external face is in contact with 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. 

 (Leyh describes this ligament as the interannular.) 



Synovial membrane. This lines the odontoid ligament, the atlo-axoid 

 ligament, 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. 



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 the occipital bone ; 2, A transverse 

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

 inferior arch of the atlas, and is attached by its extremities to the superior face of the 

 latter. A small synovial capsule facilitates 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 Carnivora 



