This is a trochoid or pivot joint, of a rather peculiar character. The
articular surfaces are: (1) On the lateral masses of the atlas, two
somewhat saddle-shaped facets, which are usually confluent ventrally;
(2) on the axis, reciprocal saddle-shaped surfaces which extend upon the
odontoid process and are confluent on its ventral aspect. It will be
observed that the joint surfaces are not at all accurately adapted to
each other, so that only limited areas are in contact at any time.
The =joint capsule= is attached around the margins of the articular
surfaces. It is loose and ample enough laterally to allow extensive
movement.
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.)
]