The Atlas is articulated with the occiput and the axis (Epistropheus).
The Articulation of the Atlas with the Occiput
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.
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.
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 :
- The anterior atlanto-epistrophic.
- The posterior atlanto-epistrophic.
- Two articular capsules (for lateral joints).
- The transverse ligament.
- 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.
The Ligaments uniting the Occiput and the Axis
The following ligaments unite bones not in contact, and are to be seen from the interior of the canal after removing the posterior arches of the epistropheus and atlas and posterior ring of the foramen magnum : -
1. The tectorial membrane.
2. The crucial ligament.
3. Two alar (or check) ligaments.
4. The apical dental ligament.
The tectorial membrane (occipito-cervical hgament) consists of a very strong band of fibres, connected below to the upper part of the body of the third vertebra and lower part of the body of the epistropheus as far as the root of the dens. It is narrow below, but widens out as it ascends, to be fastened to the basilar groove of the occiput. Laterally, it is connected with the accessory fibres of the atlanto-epistrophic capsule. It is really only the upward prolongation of the deep stratum of the posterior longitudinal ligament, the superficial fibres of which run on to the occipital bone without touching the epistropheus, thus giving rise to two strata. It is in relation in front with the crucial ligament.
The crucial ligament.
The alar (or check) ligaments are two strong rounded cords, which extend from the sides of the apex of the dens, transversely lateral- ward to the medial edge of the anterior portion of the occipital condyles.
They are to be seen immediately above the upper border of the transverse ligament, which they cross obliquely owing to its forward curve at its attachments to the atlas. Some of their fibres occasionally run across the middle line from one alar ligament to the other. At the dens they are connected with the atlan to-dental capsule, and at the condyles they strengthen the atlanto-occipital articular capsule.
The apical dental or suspensory ligament consists of a slender band of fibres ascending from the summit of the dens to the lower surface of the occipital bone, close to the foramen magnum. It is best seen from the front, after removing the anterior atlanto-occipital ligament, or from behind by drawing aside the crucial ligament.
The apical ligament is tightened by extension and relaxed by flexion or nodding; the alar ligaments not only limit the rotatory movements of the head and atlas upon the epistropheus, but by binding the occiput to the pivot, round which rotation occurs, they steady the head and prevent its undue lateral inclination upon the vertebral column. (See Transverse Ligament)
By experiments, it has been proved that the head, when placed so that the orbits look a little upward, is poised upon the occipital condyles in a line drawn a little in front of their middle; the amount of elevation varies slightly in different cases, but the balance is always to be obtained in the human body - it is one of the characteristics of the human figm'e. It serves to maintain the head erect without undue muscular effort, or a strong ligamentum nuchse and prominent dorsal spines such as are seen in the lower animals. Disturb this balance, and let the muscles cease to act, the head will either drop forward or backward according as the centre of gravity is in front or behind the balance line. The ligaments which pass over the dens to the occiput are not quite tight when the head is erect, and only become so when the head is flexed; if this were not so, no flexion would be allowed; thus, muscular action, and not liga- mentous tension, is employed to steady the head in the erect position. It is through the com- bination of the joints of the atlas and epistrophaus, and occiput and epistropheus (consisting of two paii-s of joints placed symmetrically on either side of the median line, while through the median line there passes a pivot, also with a pair of joints), that the head enjoys such freedom and celerity of action, remarkable strength, and almost absolute security against violence, which could only be obtained by a ball-and socket joint; but the ordinary ball-and-socket joints are too prone to dislocations by even moderate twists to be reliable enough when the life of the individual depends on the perfection of the articulation: hence the importance of this combination of joints.
From Morris's treatise on anatomy.