This articulation [articulatio atlanto-occipitalis] consists of a pair of joints symmetrically situated on either side of the middle line. The parts entering into their formation are the cup-shaped superior articular processes of the atlas and the condyles of the occipital bone. They are united by the following ligaments : -
1. Anterior atlanto-occipital. 3. Two articular capsules.
2. Posterior atlanto-occipital. 4. Two anterior oblique.
The anterior atlanto-occipital ligament [membrana atlanto-occipitalis anterior] is less than an inch (about 2 cm.) wide, and is composed of densely woven fibres, most of which radiate slightly lateralward as they ascend from the front surface and upper margin of the anterior arch of the atlas to the anterior border of the foramen magnum; it is continuous at the sides with the articular capsules, the fibres of which overlap its edges, and take an opposite direction medially and upward.
The central fibres ascend vertically from the anterior tubercle of the atlas to the pharyn- geal tubercle on the occipital bone; they are thicker than the lateral fibres, and are continuous below with the superficial part of the anterior atlanto-epistrophic ligarnent, and through it with the anterior longitudinal ligament of the vertebral column. It is in relation, in front, with the recti capitis anteriores; and behind, with the apical dental or suspensory ligament.
The posterior atlanto-occipital ligament is broader, more mem- branous, and not so strong as the anterior. It extends from the posterior surface and upper border of the posterior arch of the atlas to the posterior margin of the foramen magnum from condyle to condyle; being incomplete on either side for the passage of the vertebral artery into, and suboccipital nerve out of, the canal. It is somewhat thickened in the middle line by fibres, which pass from the posterior tubercle of the atlas to the lower end of the occipital crest.
It is not tightly stretched between the bones, nor does it limit their movements; it corre- sponds with the position of the ligamenta flava, but has no elastic tissue in its composition. It is in relation in front with the dura mater, which is firmly attached to it; and behind with the recti capitis posteriores minores, and enters into the floor of the suboccipital triangle. Its lateral margins, which do not reach the occipital bone but terminate on the posterior end of the superior articular processes of the atlas, form the so-called oblique ligaments of the atlas. The lateral margins of these ligaments are free and they form the posterior boundaries of the apertures through which the vertebral arteries enter and the suboccipital nerves leave the vertebral canal.
The atlanto-occipital articular capsules are very distinct and strongly marked, except on the medial side, where they are thin and formed only of short membranous fibres. They are lax, and do not add much to the security of the joint.
In front, the capsule descends upon the atlas, to be attached, some distance below the articular margin, to the front surface of the lateral mass and to the base of the transverse process ; these fibres take an obUque course upward and medialward, overlapping the anterior atlan to- occipital. At the sides and behind, the capsule is attached above to the margins of the occipital condyles; below, it skirts the medial edge of the foramen for the vertebral artery, and behind is attached to the prominent tubercle overhanging the groove for that vessel; these latter fibres are strengthened by a band running obliqviely upward and medialward to the posterior margin of the foramen magnum.
The anterior oblique or lateral occipito-atlantal ligament is an accessory band which strengthens the capsule laterally. It is an oblique, thick band of fibres, sometimes quite separate and distinct from the rest, passing upward and medialward from the upper surface of the transverse process beyond the costo-transverse foramen to the jugular process of the occipital bone.
The synovial membrane of these joints occasionally communicates with the synovial sac between the dens (odontoid process) and the transverse ligament.
The arterial supply is derived from twigs of the vertebral, and occasionally from twigs from the meningeal branches of the ascending pharyngeal.
The nerve-supply comes from the anterior division of the suboccipital nerve.
Movements. - By the symmetrical and bilateral arrangement of these joints, security and strength are gained at the expense of a very small amount of actual articular surface; the basis of support and the area of action being equal to the width between the most distant borders of the joint.
The principal movement permitted at these joints is of a ginglymoid character, producing flexion and extension upon a transverse axis drawn across the condyles at their slightly con- stricted parts.
In flexion, the forehead and chin drop, and what is called the nodding movement is made; in extension, the chin is elevated and the forehead recedes.
There is also a slight amount of gliding movement, either directly lateral, the lateral edge of one condyle sinking a little within the lateral edge of the socket of the atlas, and that of the opposite condyle projecting to a corresponding degree. The head is thus tilted to one side, and it is even possible that the weight of the skull may be borne almost entuely on one joint, the articular surfaces of the other being thrown out of contact.
Or the movement may be obliquely lateral, when the lower side of the head will be a trifle
in advance of the elevated side. In this motion, which takes place on the antero-posterior axis, one condyle advances slightly and approaches the middle line, while the other recedes. This is of the nature of rotation, though there is no true rotation round a vertical axis possible between the occiput and atlas.
These lateral movements are checked by the alar ligaments and the lateral part of the capsules; extension is checked by the anterior atlanto-oocipital and anterior oblique ligaments, and flexion by the posterior part of the capsule and the tectorial membrane.
Muscles acting upon the occipito-atlantal joint. - Flexion whereby the chin is approxi- mated toward the sternum is produced by the weight of the anterior part of the head and by all muscles which are attached to the hyoid bone or to the bones of the skuU in front of a trans- verse axis between the two condyles. These muscles take their fixed point below either from the vertebral columir, the sternum, or the bones of the shoulder girdle. Before those connected with the mandible can act that bone must be fixed by the muscles of mastication which, there- fore, also take part in the movements. It must be noted that the sterno-mastoid muscles are powerful flexors, although a part of their insertion is behind the transverse axis between the two condyles.
Extension is due to the action of muscles or portions of muscles inserted into the skull behind the transverse axis above mentioned, and connected below either with the vertebral column, shoulder girdle, or sternum.
Lateral movement is produced by the anterior and posterior groups of muscles on the same side acting simultaneously and aided by the rectus capitis lateralis of that side.
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