The Articulations between the Atlas and Epistropheus (Axis)

1. The Lateral Atlanto-epistrophic Joints.

2 . The Central Atlanto-epistrophic Joint or the Atlanto-dental.

Class. - Diarthrosis. Subdivision. - Arthrodia. Class. - Diarthrosis. \ Subdivision. - Trochoides.

The bones that enter into the formation of the lateral joints are the inferior articular processes of the atlas and the superior of the epistropheus (axis); the central joint is formed by the dens (odontoid process) articulating in front with the atlas, and behind with the transverse ligament.

The ligaments which unite the epistropheus and atlas are :

1. The anterior atlanto-epistrophic.
2. The posterior atlanto-epistrophic.
3. Two articular capsules (for lateral joints).
4. The transverse ligament.
5. The atlanto-dental articular capsule.

The anterior atlanto-epistrophic ligament is a narrow but strong membrane filling up the interval between the lateral joints. It is attached above to the front surface and lower border of the anterior arch of the atlas, and below to the transverse ridge on the front of the body of the epistropheus. Its fibres are vertical, and are thickened in the median line by a dense band which is a continuation upward of the anterior longitudinal ligament of the vertebral column.

This band is fixed above to the anterior tubercle of the atlas, where it becomes continuous with the central part of the anterior atlanto-oocipital ligament; it is sometimes sepa- rated by an interval from the deeper ligament, and is often described as the superficial atlanto- epistrophic ligament. It is in relation with the longus colli muscle.

The posterior atlanto-epistrophic ligament is a deeper, but thinner and looser membrane than the anterior. It extends from the posterior root of the transverse process of one side to that of the other, projecting laterally beyond the posterior part of the capsules which are connected with it. It is attached above to the posterior surface and lower edge of the posterior arch of the atlas, and below to the superior edge of the laminae of the epistropheus on their dorsal aspect.

It is denser and stronger in the median line, and has a layer of elastic tissue on its anterior surface like the ligamenta flava, to which it corresponds in position. It is connected in front with the dura mater; behind, it is in relation with the inferior oblique muscles, and is perforated at each side by the second cervical nerve.

1. The Lateral Atlanto-epistrophic Joints are provided with short, ligamentous fibres, forming ari:icular capsules, which completely sur- round the lateral articular facets. Lateral to the canal they are attached some little distance from the articular margins, extending along the roots of the transverse processes of the epistropheus nearly to the tips, but between the roots they skirt the medial edge of the costo-transverse foramina. They are strength- ened in front and behind by the atlanto-epistrophic hgaments.

Medially each capsule is thinner, and attached close to the articular mai'gins, being strength- ened behind by a strong band of slightly oblique fibres passing upward along the lateral edge of the tectorial membrane from the body of the epistropheus to the lateral mass of the atlas behind the transverse ligament; some of these fibres pass on, thickening and blending with the atlanto- oocipital capsule, to be inserted into the margin of the foramen magnum. This band is some- times called the accessory band.

There is a synovial membrane for each joint.

2. The Central Atlanto-epistrophic Joint, although usually described as one, is composed of two articulations, which are quite separate from one another: an anterior between the dens and the arch of the atlas, and a posterior between the dens and the transverse ligament.

The transverse ligament is one of the most important structures in the body, for on its integrity and that of the alar ligaments our lives largely depend. It is a thick and very strong band, as dense and closely woven as fibro-cartilage, about a quarter of an inch (6 mm.) deep at the sides, and somewhat more in the middle line. Attached at each end to a tubercle on the inner side of the lateral mass of the atlas, it crosses the ring of this bone in a curved manner, so as to have the concavity forward; thus dividing the ring into a smaller anterior portion for the dens and a larger posterior part for the spinal cord and its membranes, and the spinal accessory nerves.

It is flattened from before backward, being smooth in front, and covered by synovial mem- brane to allow it to glide freely over the posterior facet of the dens. Where it is attached to the atlas it is smooth and well rounded off to provide an easy floor of communication between the transverso-dental and occipito-atlantal joints.

To its posterior surface is added, in the middle line, a strong fasciculus of vertical fibres, passing upward from the root of the dens to the basilar border of the foramen magnum on its cranial aspect. Some of these fibres are derived from the transverse ligament. These vertical fibres give the transverse liga- ment a cruciform appearance; hence the name, the crucial ligament applied to the whole.

The atlanto-dental articular capsule is a tough, loose membrane, completely surrounding the apposed articular surfaces of the atlas and dens.

At the dens it blends above with the front of the alar and central occipito-odontoid liga- ments, and arises also along the sides of the articular facet as far as the neck of the dens; the fibres are thick, and blend with the capsules of the lateral joint. At the atlas they are attached to the non-articular part of the anterior arch in front of the tubercles for the transverse liga- ment, blending, above and below the borders of the bone, with the anterior atlanto-occipital and atlanto-epistrophic ligaments, as well as with the medial portion of the articular capsules. It holds the dens to the anterior arch of the atlas after aU the other ligaments have been divided.

The synovial membranes are two in number: - one for the joint between the dens and atlas; and another (transverso-dental) for that between the transverse ligament and the dens. This last often communicates with the atlanto-occipital articulations; it is closed in by membranous tissue between the borders of the transverse ligament and the margin of the facet on the dens, and is separated from the front sac by the atlanto-dental articular capsule.

The arterial supply is from the vertebral artery, aiid the nerve-supply from the loop between the first and second cervical nerves.

Movements. - The chief and characteristic movement at these joints is the rotation, in a nearly horizontal plane, of the collar formed by the atlas and transverse ligament, round the dens as a pivot, which is extensive enough to allow of an all-round view without twisting the trunk. Partly on account of its ligamentous attachments, and partly on account of the shape of the articular siirfaces, the cranium must be carried with the atlas in these movements. The rotation is checked by the ligaments passing from the dens to the occiput (alar ligaments), and also by the atlanto-epistrophic. Owing to the fact that the facets of both atlas and epistropheus, which enter into the formation of the lateral atlanto-epistrophic articulations, are convex from before backward, and have the articular cartilage thicker in the centre than at the circumfer- ence, the motion is not quite horizontal but slightly curvilinear. In the erect position, with the face looking directly forward, the most convex portions of the articular surfaces are alone in contact, there being a considerable interval between the edges; dm-ing rotation, therefore, the prominent portions of the condyles of the atlas descend upon those of the epistropheus, dimin- ishing the space between the bones, slackening the ligaments, and thus increasing the amount of rotation, without sacrificing the security of the joint in the central position.

Besides rotation, forward and backward movements and some lateral flexion are permitted between the atlas and epistropheus, even to a greater extent than in most of the other vertebral joints.

The muscles acting upon the atlanto-epistrophic joints. - The muscles capable of producing rotation at the atlanto-epistrophic joints are those which take origin from near the mesial plane either in front or behind and which are attached above either to the atlas or the skull, lateral to the atlanto-epistrophic joints. When the muscles which lie at the back of the joint on one side act they will turn the head to the same side and will be aided by the muscles in front on the opposite side. If the muscles in front and behind on the same side act simultaneously, they will pull down the head to that side and will be aided by muscles which pass more or less vertically from the transverse process of the atlas to points below.