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Lig. Obturatorium Atlantoepistrophica Posterior. The posterior covering atlanto-epistropheal ligament (O.T. posterior atlo-axoid ligament) (Fig. 296) occupies the position which is elsewhere taken by the ligamenta flava. It extends from the posterior arch of the atlas to the upper border of the vertebral arch of the epistropheus.
Lig. Transversum Atlantis.-The transverse ligament of the atlas (Figs. 296 and 297) is a strong band, placed transversely, which arches backwards behind the neck of the dens of the epistropheus. By its extremities it is attached to the tubercle on the medial aspect of each lateral mass of the atlas. A thin plate of fibro-cartilage is developed in its central part.
A stratum synoviale (synovial membrane) lines each of the three articular capsules, and in addition a synovial sac is developed between the dens and the lig. transversum atlantis. This is more extensive than the synovial cavity between the dens and the atlas.
There are two articulations between the atlas and the occipital bone. Each is a diarthrosis in which movement takes place in relation to two axes, viz. the
FIG. 297.-DISSECTION FROM BEHIND OF THE LIGAMENTS CONNECTING THE OCCIPITAL BONE, THE ATLAS, AND THE EPISTROPHEUS WITH EACH OTHER.
transverse and the antero-posterior. The condyle of the occipital bone is biconvex, and fits into the bi-concave superior articular surface of the atlas, while the long axes of the two joints are directed horizontally forwards and medially.
Ligamenta.-Each articulation is provided with a capsula articularis which is thin but complete. It is attached to the rough non-articular surfaces surrounding the articular areas on the atlas and occipital bone.
The following supplementary ligaments are the chief structures which bind the atlas to the occipital bone :
The membrana atlanto-occipitalis anterior (anterior occipito-atloid membrane) (Fig. 296) is a strong although thin membrane, attached inferiorly to the anterior arch of the atlas, and superiorly to the anterior half of the circumference of the foramen magnum. Laterally it is in continuity with the articular capsules, while in the median plane, where it extends from the anterior tubercle of the atlas to the basilar part of the occipital bone, it presents a specially well-defined thickened band which might be regarded as a separate accessory ligament or as the beginning of the anterior longitudinal ligament of the vertebræ.
The membrana atlanto-occipitalis posterior (posterior occipito-atloid membrane) (Fig. 296) is another distinct but still thin membrane which is attached superiorly to the posterior half of the circumference of the foramen magnum, and inferiorly to the upper border of the posterior arch of the atlas. Laterally it also is continuous with the articular capsules. On each side of the median plane its inferior border
is arched in relation to the vertebral groove, and is therefore to some extent free, in order to permit the passage of the posterior ramus of the first cervical nerveand the vertebral artery. Not infrequently this arched border becomes ossified, thus converting the groove on the bone into a foramen.
A synovial stratum lines each of the articular capsules.
There is no direct articulation between the epistropheus and the occipital bone, but union between them is effected by means of the following accessory ligaments:
The membrana tectoria (Fig. 296) is situated within the vertebral canal, and is usually regarded as the upward continuation of the posterior longitudinal ligament of the vertebral bodies. It extends from the posterior surface of the body of the epistropheus to the basilar groove on the superior surface of the basilar part of the occipital bone, spreading laterally on the circumference of the foramen magnum. Some of its deepest fibres are attached to the atlas immediately above the atlantoepistropheal articulation.
Subjacent to the membrana tectoria there is the ligamentum cruciatum atlantis (Fig. 297), a structure which is very closely associated with the lig. transversum atlantis. It consists of a crus transversum, formed by the superficial fibres of the transverse ligament of the atlas; a crus inferius, consisting of median longitudinal fibres which are attached below to the posterior surface of the body of the epistropheus, and above to the crus transversum; and a crus superius, also median and longitudinal, whose fibres extend from the crus transversum upwards to the posterior surface of the basilar part of occipital bone, immediately subjacent to the membrana tectoria.
Ligamenta Alaria.-The alar ligaments (O.T. check ligaments) (Fig. 297) are two very powerful, short, and somewhat rounded bands. They are attached medially to the sides of the summit of the dens, and laterally to the tubercle on the medial aspect of the condylar portions of the occipital bone.
Ligamentum Apicis Dentis.-The ligament of the apex of the dens (O.T. middle odontoid) (Fig. 297) consists of fibres running vertically upwards from the apex of the dens to the median part of the anterior margin of the foramen magnum. This ligament to some extent represents an intervertebral fibro-cartilage, in the centre of which remains of the notochord may be regarded as present.
Even in advanced life a small lenticular mass of cartilage, completely surrounded by bone, persists in the plane of fusion between the dens and the body of the epistropheus.
Movements at these Joints.At the joints between occipital bone and atlas the movements are very simple, and consist essentially of movements whereby the head is elevated and depressed upon the vertebral column (nodding movements). In addition a certain amount of oblique movement is possible, during which great stability is attained by resting the anterior and posterior parts of opposite condyles upon corresponding parts of the atlas.
The head and the atlas rotate together upon the epistropheus, the pivot of rotation being the dens, and the amount of rotation is limited by the ligamenta alaria. No rotation can occur between the occiput
FIG. 298.-MANDIBULAR JOINT.
and atlas, and stability between atlas and epistropheus is best attained after a slight amount of rotation, similar to the oblique movement between occipital bone and atlas.
The mandibular joint (O.T. temporo-mandibular) is an arthrodial diarthrosis. It occurs between the mandibular fossa of the temporal bone and the condyle of the mandible. These two articular surfaces are markedly dissimilar both in size and shape. In its general outline the articular surface of the head of the mandible is cylindrical, having its long axis directed from the medial side laterally and forwards. On the other hand, the mandibular fossa is concavo-convex from behind forwards. Its articular surface includes the tuberculum articulare-the eminence at the base of the anterior root of the zygoma. The articular surfaces of the bones are clothed with hyaline encrusting cartilage, whilst the articular cavity is divided into a superior and inferior part by a disc of fibro-cartilage.
Ligaments.-The joint is invested by an articular capsule which is quite complete, but is very thin on the medial side. The lateral part of the fibrous stratum of the capsule-the temporo-mandibular liga
FIG. 299.-SECTION THROUGH THE MANDIBULAR
ment (O.T. external lateral) (Fig. 298)-is divisible into anterior and posterior portions which are attached superiorly to the root tubercle and inferior border of the zygomatic process of the temporal bone, and inferiorly to the lateral side and posterior border of the neck of the mandible. The direction of its fibres, is downwards and backwards.
Within the capsule there is a disc of fibro-cartilage, the discus articularis (Fig. 299), which is moulded upon the condyle of the mandible below, and on the articular surface of the temporal bone above. It thus compensates for the incongruity between the articular surfaces of the two bones. The disc is attached circumferentially to the capsule. It is widest in the transverse direction, thicker posteriorly than anteriorly, and thinnest towards the centre, where it may be perforated. Its anterior margin is intimately associated with the insertion of the external pterygoid muscle.
A synovial stratum lines each of the compartments into which the joint cavity is divided by the disc. As a rule these membranes are separate from each other, but they become continuous when the disc is perforated. The superior synovial stratum is larger and more loosely disposed than the lower.
FIG. 300.-SPHENO-MANDIBULAR LIGAMENT OF THE
Situated on the medial aspect of the joint, but at a short distance from it, and quite distinct from the capsule, there is an accessory band called the lig. sphenomandibulare (Fig. 300). Superiorly the spheno-mandibular ligament (O.T. internal lateral) is attached to the angular spine of the sphenoid bone, and inferiorly to the inferior as well as the anterior border or lingula of the inferior alveolar foramen. It is not an articular ligament in the true sense; for, instead of being connected with the joint, it is developed in the tissue surrounding part of Meckel's cartilage.
Portions of the following structures are found in the interval between the spheno-mandibular ligament and the ramus of the mandible-viz., the external pterygoid muscle; internal maxillary vessels; inferior alveolar vessels and nerve; middle meningeal vessels; auriculotemporal nerve; and sometimes a deep portion of the parotid gland.
Movements of the Mandible. The nature of the movements which the mandible can perform is determined partly by the character of the articular surfaces of the mandibular joint, and partly by the fact that, while the two joints always act simultaneously, they may also, to some extent, perform the same movement alternately.
When movement takes place through the long or transverse horizontal axis of each joint, the mandible may be elevated, as in clenching the teeth, or it may be depressed, as in gaping. In the latter movement the condyle leaves the mandibular fossa, and, along with the disc, it moves forwards until they rest upon the tuberculum articulare. Meantime the chin describes the arc of a circle, of which the centre or point of least movement corresponds to the position of the inferior alveolar foramen, and thus the structures which enter at that foramen are protected against stretching. Coincidently with the forward movement of the condyle, it glides in a revolving manner upon the inferior aspect of the disc.
At any stage in the movement of depressing the chin the mandible may be protruded, so that the inferior incisor teeth are projected in front of the upper set, a movement which results from the condyles of the mandible being drawn forwards upon the articular tubercles. A similar relation of the condyle to the articular tubercle occurs during the exaggerated depression of the mandible which results from yawning, in which position the articulation is liable to be dislocated. When the two joints perform the same movement alternately, a certain amount of lateral motion results, from the fact that the long axis of each joint presents a slight obliquity to the transverse axis of the skull, and consequently a grinding or oblique movement in the horizontal plane is produced. Excessive depression, with the risk of dislocation, is resisted by the fibres of the temporo-mandibular ligament, which becomes tense.
In all movements of the mandible the disc conforms closely to the position of the condyle, and they move forwards and backwards together, but at the same time the disc does not restrict the movements of the condyle. Thus while the disc, along with the condyle, is gliding upon the temporal aspect of the joint, the condyle itself revolves upon the inferior surface of the disc.
CRANIAL LIGAMENTS NOT DIRECTLY ASSOCIATED WITH ARTICULATIONS.
Lig. Stylomandibulare. The stylo-mandibular ligament (Figs. 298 and 300) is a specialised portion of the deep cervical fascia which extends from the anterior aspect of the tip of the styloid process of the temporal bone to the posterior border of the angle of the mandible, between the insertions of the masseter and internal pterygoid muscles.
Lig. Pterygospinosum.-The pterygo-spinous ligament is a membrane extending from the upper part of the posterior free margin of the lateral pterygoid lamina, posteriorly and slightly laterally, to the angular spine of the sphenoid. An interval is left between its upper border and the floor of the skull for the outward passage of those branches of the inferior maxillary nerve which supply the external pterygoid, temporal, and masseter muscles. This ligament has a tendency to ossify either wholly or partially.
Lig. Stylohyoideum. The stylo-hyoid ligament may be regarded as the downward continuation of the styloid process of the temporal bone. Inferiorly it is attached to the lesser cornu of the hyoid bone. It is not infrequently ossified, in which case it constitutes the epihyal bone found in many animals.
THE JOINTS OF THE THORAX.
Articulationes Costovertebrales (Costo - vertebral Articulations). The typical rib articulates with the vertebral column both by its head and by its tubercle. Thus, two sets of articulations, with their associated ligaments, exist between the ribs and the vertebræ, but each set is constructed upon a common plan, with the exception of certain joints situated at the upper and lower ends of the series, where the ribs themselves deviate from the typical form.
The articulations of the heads of the ribs with the bodies of the vertebræ (Fig. 293) are all diarthroses, which, from their somewhat hinge-like action, may be classed as ginglymoid.
The head of every typical rib is wedge-shaped, and presents two articular facets, an upper and a lower, separated from each other by an antero-posterior ridge which abuts against an intervertebral fibro-cartilage, while the articular facets articulate with similar surfaces on the contiguous margins of the two vertebræ adjoining the fibro-cartilage. These surfaces form a wedge-shaped depression or cup, the bottom of which is more elastic than the sides, and thus an arrangement is provided which tends to reduce the shock of blows upon the walls of the chest.
Each of these articulations is provided with an articular capsule which surrounds and encloses the joint, and is attached to contiguous non-articular margins on the head of the rib and the two vertebral bodies. On its anterior or ventral aspect the capsule presents three radiating fasciculi which collectively form the lig. capituli costa radiatum (radiate ligament of the head of the rib (O.T. stellate)) (Fig. 293). These fasciculi radiate from a centre on the anterior surface of the head of the rib, so that the middle fasciculus becomes attached to the intervertebral fibro-cartilage while the upper and lower fasciculi proceed to the adjacent margins of the two vertebræ between which the fibro-cartilage is situated, and with which the rib articulates. To a slight extent these radiating fasciculi pass under cover of the lateral margin of the anterior longitudinal ligament of the vertebral bodies. In those joints in which the head of the rib does not articulate with an intervertebral fibro-cartilage the central fasciculus of the radiate ligament is wanting, but the other two retain the same general arrangement.
Lig. Capituli Costa Interarticulare. The interarticular ligament of the head of the rib consists of short transverse fibres within the capsule. These are attached, on the one hand, to the ridge which intervenes between the two facets on the head of the rib, and on the other to the lateral aspect of the intervertebral fibro-cartilage. This ligament is not a disc or meniscus, but merely an interarticular ligament, of width sufficient to divide the joint cavity into an upper and a lower compartment. It is absent from those joints which do not articulate with an intervertebral fibrocartilage, i.e. from those ribs which articulate with the body of only one vertebra.
The interarticular ligament is supposed to represent the lateral end of a ligament which, under the name of the lig. conjugale costarum, connects the heads of the ribs of certain mammals across the posterior aspect of the intervertebral fibro-cartilage, and, in the human subject, until the seventh month of fœtal life, connects the posterior aspects of the necks of a pair of ribs with each other across the median plane.
A stratum synoviale lines each joint cavity, and therefore, in all cases where the joint is divided into two compartments, each one has its own synovial lining.
In the costo-transverse joints the tubercle of each typical rib articulates with the transverse process of the lower of the two thoracic vertebræ with which the head of the rib is associated. Near the tip of the transverse process there is an articular facet, on its anterior aspect, for articulation with the corresponding facet on the medial articular part of the rib tubercle. The joint so formed is an arthrodial diarthrosis.
The joint cavity is surrounded by a comparatively feeble capsula articularis, which is attached immediately beyond the margins of the articular facets, and in which no special bands can be distinguished.
A simple stratum synoviale lines the capsule in all cases where the latter is present.
The following accessory ligaments, in connexion with this joint, strengthen and support the articulation:
Ligamentum Costotransversarium Anterius.-The anterior costo-transverse ligament (O.T. superior) (Fig. 293) consists of strong bands of fibres which are attached to the superior border of the neck of the rib, extending from the head laterally to the non-articular part of the tubercle. All these fibres may be traced upwards. Those situated nearest to the head of the rib proceed obliquely upwards and laterally, to be attached to the transverse process immediately above, but with