The =superior atlanto-axial ligament= (Ligamentum interarcuale) is membranous and reinforces the capsule dorsally. The =interspinous ligament= (Ligamentum inter-spinale) consists of two elastic bands which extend from the dorsal arch of the atlas to the spine of the axis. The =inferior atlanto-axial ligament= (Ligamentum dentis externum) arises from the ventral tubercle of the atlas and is attached by two branches on the ventral spine of the axis. The =odontoid ligament= (Ligamentum dentis internum) is short, very strong, and somewhat fan-shaped. It extends from the rough concave dorsal surface of the dens or odontoid process, widens in front, and is attached to the transverse rough area on the inner surface of the ventral arch of the atlas. =Movements.=—The atlas and the head rotate upon the axis; the axis of rotation passes through the center of the odontoid process and body of the axis. [Illustration: FIG. 141.—ATLANTO-OCCIPITAL AND ATLANTO-AXIAL JOINTS OF HORSE, DORSAL VIEW AFTER REMOVAL OF DORSAL ARCH OF ATLAS. _a_, Joint capsule of left part of atlanto-occipital joint; _b_, lateral ligament of same; _c_, _c′_, odontoid ligament; _d_, atlanto-occipital joint capsule; _e_, joint capsule of articulation between axis and third cervical vertebra; _f_, interspinous ligament; _1_, occipital bone; _2_, atlas; _3_, axis; _4_, third cervical vetrebra. (Ellenberger-Baum, Anat. d. Haustiere.) ] THE ATLANTO-OCCIPITAL ARTICULATION This joint may be classed as a ginglymus. The =articular surfaces= of this joint are: (1) On the atlas, two deep oval cavities; (2) the corresponding condyles of the occipital bone. The joint surfaces are oblique, coming very close to the median line ventrally, but separated by a considerable interval dorsally. Posteriorly, a triangular rough area cuts into the middle of each articular surface on the atlas. There are two roomy =joint capsules=, which sometimes communicate ventrally, especially in old subjects. The =superior atlanto-occipital membrane= extends from the dorsal arch of the atlas to the upper margin of the foramen magnum. It is blended with the capsules and contains many elastic fibers. The =inferior atlanto-occipital membrane= extends from the ventral arch of the atlas to the lower margin of the foramen magnum. It is narrower and thinner than the superior membrane, and also fuses with the joint capsules. The =lateral ligaments= are two short bands which are partially blended with the capsules. Each is attached to the border of the wing of the atlas near the intervertebral foramen, and to the outer surface of the paramastoid or styloid process of the occipital bone. =Movements.=—These are chiefly flexion and extension. A small amount of lateral oblique movement is also possible. ARTICULATIONS OF THE THORAX COSTO-VERTEBRAL ARTICULATIONS