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.