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the interior of an articular cavity except the articular encrusting cartilage. Consequently the synovial stratum is absent only from the free surface of articular cartilage, although it forms a thicker layer upon the inner surface of the articular capsule than upon the free surfaces of interarticular ligaments, discs, and menisci.
It is not necessary to suppose that the synovial stratum has disappeared from these articular cartilages as the result of friction, because, notwithstanding constant friction, such parts as the interior of articular capsules or the menisci of the knee-joint have not been denuded of their synovial covering.
As the epiphyses adjoining articular cavities are produced in the joint-units, the attachments of the capsule should be found upon, and restricted to, the non-articular surfaces of the articular epiphyses. While this is the case in their earliest stages, yet, as development advances, considerable variations arise, until, in the adult condition, the capsule of the larger articulations, more particularly of the extremities, is not always restricted to the epiphyses for its attachments. The student will readily perceive and appreciate these variations by comparing the accounts and illustrations of the epiphyses with those of the articulations, and he should note that in some cases the epiphysial line is extra-capsular, i.e. the capsular attachment is restricted to the epiphysis; in some the line is intra-capsular; and in some the epiphysial line is partly intra-capsular and partly extra-capsular.
MORPHOLOGY OF LIGAMENTS.
From what has been said in connexion with the development of joints, it will be evident that ligaments are essentially products derived from the cellular articular disc.
Nevertheless, in relation to the fully formed joint, many structures are described as ligaments which do not take origin in the manner just indicated. Some of these ligamentous structures remain fairly distinct from the articular capsules with which they are immediately associated; others become thoroughly incorporated with the articular capsules and cannot be separated therefrom, while yet others may be found situated within the capsule of a joint, and thus play the part of interarticular ligaments.
Instances of each of these forms of adventitious ligaments may be readily given. For example, we may instance the expansion of the tendon of the semimembranosus muscle to the oblique ligament of the knee-joint, and the offshoots from the tendon of the tibialis posterior muscle to the plantar aspects of various tarsal bones, as illustrations of structures which play an important part as ligaments, but are not indelibly incorporated with the joint capsule.
Of structures which have become indelibly incorporated with the primitive capsule, we may instance the broad tendinous expansions of the quadriceps extensor muscle around the knee-joint. The tibial collateral ligament of the same joint is regarded as a detached portion of the tendon belonging to that part of the adductor magnus muscle which takes origin from the ischium, while the fibular collateral ligament of the knee is considered by some to be the primitive femoral origin of the peronaeus longus musele. Another illustration of the same condition is found in the coraco-humeral ligament, which is regarded by some as representing a detached portion of the pectoralis minor muscle.
Two illustrations may be given of structures playing the part of ligaments within the capsule of a joint, although in the first instance they are not developed as ligaments. It is questionable if the ligamentum teres of the hip-joint is an interarticular ligament in the true ense of the term; it has been regarded as the isolated and displaced tendon of the ambiens muscle found in birds. In the shoulder-joint, many observers look upon the superior gleno-humeral ligament as representative of the ligamentum teres.
Such structures as the stylo-hyoid ligament and the spheno-mandibular ligament, although described as ligaments, are in reality skeletal parts which have not attained their complete ssific development.
Again, certain portions of the deep or muscular fascia of the body which become specialised into restraining and supporting bands (e.g. the ilio-tibial tract of the fascia lata; the stylo-mandibular ligament; the transverse carpal and dorsal carpal ligaments of the wrist-joint; the transverse crural ligament, and lig. laciniatum of the ankle-joint), although called ligaments, have no direct developmental association with articular ligaments.
Lastly, the inguinal ligament of Poupart and the lacunar ligament of Gimbernat, being special developments in connexion with an expanded tendon or aponeurosis, are still further removed from association with an articulation.
LIGAMENTA COLUMNÆ VERTEBRALIS ET CRANII.
Ligaments of the Vertebral Column and Skull.-All vertebræ, with the exception of those which deviate from the common vertebral type, present two sets of articulations whose various parts are arranged upon a uniform pattern. Thus, every pair of typical vertebræ presents an articulation between the bodies and a pair of articulations between the vertebral arches. With the latter there
are associated various important accessory ligaments which bind together laminæ, spinous processes, and transverse processes.
Articulations between Bodies of Vertebræ. These are amphiarthrodial joints. Singly, they present only a slight degree of mobility, but when this amount of movement is added to that of the whole series, the range of movement of the vertebral
FIG. 292.-MEDIAN SECTION THROUGH A PORTION OF THE LUMBAR PART OF THE VERTEBRAL COLUMN. column becomes considerable. The articular surfaces are the flattened surfaces of adjacent vertebral bodies. They are bound together by the following structures:Fibrocartilagines Intervertebrales (Fig. 292).-Each intervertebral fibrocartilage accommodates itself to the space it occupies between the two vertebral bodies, to both of which it is firmly adherent. The fibro-cartilages, from different
FIG. 293.-ANTERIOR LONGITUDINAL LIGAMENT OF THE VERTEBRAL COLUMN, AND THE COSTO-VERTEBRAL JOINTS AS SEEN FROM THE FRONT.
parts of the vertebral column, vary in vertical thickness, being thinnest from the third to the seventh thoracic vertebra, and thickest in the lumbar region. In the cervical and lumbar regions each fibro-cartilage is thicker anteriorly than posteriorly, thereby assisting in the production of the anterior convexity which characterises the vertebral column in these two regions. In the thoracic region
the fibro-cartilages are thinnest on their anterior aspects in correspondence with the anterior concavity of this section of the vertebral column.
Each fibro-cartilage consists of a circumferential portion, annulus fibrosus, formed for the most part of oblique parallel fibres running from one vertebra to the other; horizontal fibres are also found. The axial or central part of the fibrocartilage, the nucleus pulposus, is elastic, soft, and pulpy.
The superior and inferior surfaces of the fibro-cartilage are closely adherent to the adjoining epiphyseal plates of the vertebral bodies, and as ossification advances, the distinction between epiphyseal plates and vertebral body disappears. As a rule the transverse diameter of the fibro-cartilage corresponds to that of the vertebral bodies which it joins together; but in the cervical region, where the inferior margin of the super-imposed vertebra is overlapped on each side by the one which bears it, the fibro-cartilage does not extend to the extreme lateral margin, and in this position a small diarthrosis may be seen at each lateral margin of the fibro-cartilage.
Lig. Longitudinale Anterius.-The anterior longitudinal ligament (O.T. anterior common ligament) (Fig. 293) consists of a wide stratum of longitudinal fibres which extends from the front of the epistropheus vertebra to the front of the superior segment of the sacrum, and becomes gradually wider from above downwards. It lies on the anterior surfaces of the intervertebral fibro-cartilages, to which it is firmly attached as it passes from one vertebra to the other. Its fibres vary in length. Some are attached to contiguous margins of two adjoining vertebræ; others pass in front of one vertebra to be attached to the next below, and yet others find their lower attachment three or four vertebræ below the one from which they started. None of the fibres are attached to the transverse depression on the anterior surface of a vertebral body.
Lig. Longitudinale Posterius. The posterior longitudinal ligament (O.T. posterior common ligament) (Fig. 294) is found within the vertebral canal upon the posterior aspect of the vertebral bodies. It
consists of longitudinal fibres, and it extends from the sacrum to the epistropheus vertebra, superior to which it is continued to the skull as the membrana tectoria. Opposite each intervertebral fibro-cartilage it is attached to the entire width of the adjacent margins of the two vertebral bodies, its fibres being continued over the posterior surface of the fibro-cartilage. In the lumbar and thoracic regions the width of the ligament is considerably reduced opposite the back of each vertebral body, and thus it forms a series of dentate projections along both of its margins; but in the cervical region the width of the ligament is more uniform. One or two large thin-walled veins escape from the body of each vertebra under cover of this ligament.
Articulations between Vertebral Arches.The vertebral arch of each typical vertebra carries two pairs of articular processes, by means of which it articulates with adjacent vertebral arches. The articulations between these processes are true diarthroses of the arthrodial variety.
Root of vertebral arch divided
Posterior longitudinal ligament
The distinctive characters of these articular surfaces, as regards their shape and direction in the different groups of vertebræ, have been referred to in the section on osteology.
All these articulations are provided with complete but very thin-walled capsulæ articulares, which are thinnest and loosest in the cervical region, where also the movements are freest. Each capsule is lined with a stratum synoviale.
Associated with these joints between vertebral arches are certain ligaments which are accessory to the articulations, although they are quite distinct from the capsule.
The laminæ of adjoining vertebræ are bound together by the ligamenta flava (O.T. subflava) (Fig. 295), which consist of yellow elastic fibres. The ligamenta flava close the vertebral canal in the intervals between the laminæ. Each ligament
is attached superiorly to the anterior aspect of one lamina at a short distance above its inferior border, and inferiorly it is attached to the posterior aspect of the subjacent lamina.
In the thoracic region, where the imbrication of adjoining lamina is a prominent feature, these ligaments are not so distinctly visible from behind as they are in the regions where imbrication of the lamina is not so marked.
Laterally they extend as far as the articular capsules, while medially the margins of the ligaments of opposite sides meet under cover of the root of the spinous process.
Contiguous spinous processes are also attached to each other by ligamenta interspinalia (interspinous ligaments) (Fig. 292). These are strongest in the lumbar, and weakest in the thoracic region. Each consists of layers of obliquely interlacing fibres which spring from near the tips of the two adjacent spinous process
FIG. 295. LIGAMENTA FLAVA AS SEEN FROM THE FRONT AFTER RE-
and radiate to their opposing margins. In the anteroposterior direction they extend from the base to the tip of the spinous process.
The ligamenta supraspinalia (supra-spinous ligaments) (Fig. 292) consist of longitudinal bands of fibres of varying lengths. They extend from spine. to spine, being attached to their tips, and are situated superficial to, although in continuity with, the ligamenta interspinalia.
In the cervical region this series of ligaments is extensively developed, where they project backwards from the spinous processes between the muscles of the two sides of the neck in the form of an elastic partition called the ligamentum nucha.
The antero-posterior extent of the ligamentum nuchæ increases as it approaches the occiput, where it is attached to the external occipital crest from the external occipital protuberance to the posterior border of the foramen magnum. Its posterior margin is free, and extends from the external occipital protuberance to the spine of the vertebra prominens.
Between the transverse processes there are ligamenta intertransversaria, which consist of vertical fibres extending from the postero-inferior aspect of one transverse process to the superior margin of that next below. These ligaments are generally absent from the cervical and upper thoracic regions.
Sacro-coccygeal Symphysis. The last piece of the sacrum is joined to the first piece of the coccyx by an intervertebral fibro-cartilage, and the junction is rendered more secure by the presence of certain strong ligaments. A lig. sacrococcygeum anterius, continuous with the lig. longitudinale anterius, is placed in front. A lig. sacrococcygeum posterius, which stretches downwards from the sharp border of the lower opening of the sacral canal, strengthens the joint behind. A
lig. sacrococcygeum laterale supports the joint on each side, whilst strong bands pass between the cornua of the two bones and constitute the interarticular ligaments. Intercoccygeal Joints.-So long as they remain separate, the different pieces of the coccyx are joined by intervertebral fibro-cartilages and by anterior and posterior ligaments.
Movements of the Vertebral Column.-Although the amount of movement permissible between any two vertebræ is extremely limited, yet the total range of movement capable of being attained by the entire vertebral column is very considerable.
Flexion may occur both forwards and backwards at the articulations of vertebral bodies, but more freely in the lumbar and cervical regions than in the thoracic region, where the limited amount of intervertebral fibro-cartilage and the imbrication of the lamina and spines restrict the movement. Backward flexion is most pronounced in the cervical region, and forward flexion in the lumbar region. Between the articular surfaces of the articulations between vertebral arches a variety of movements are permitted, dependent upon the directions of these surfaces. Thus lateral flexion is permitted in the lumbar, but not in the cervical or dorsal regions. Again, in the lumbar region rotation does not occur, owing to the shape of the articular processes, while it is possible in the thoracic region. In the cervical region the shape and position of the articular surfaces prevent the occurrence both of lateral flexion and of rotation as isolated movements, but a combination of these two movements may take place, whereby rotatory movement in an oblique median axis results. Finally, in the lumbar region, by combining the four forms of flexion, viz., forward, backward, and lateral, a certain amount of circumduction is possible.
Between the atlas and epistropheus vertebræ three diarthroses occur. Two of them are situated laterally, in relation to the articular processes, and are called
FIG. 296.-MEDIAN SECTION THROUGH THE ATLANTO-OCCIPITAL AND ATLANTO-EPISTROPHEAL JOINTS. arthrodial diarthroses, because of the flattened nature of the articulating surfaces. The third articulation is median in position. It is found between the smooth anterior surface of the dens of the epistropheus and the articular facet on the posterior aspect of the anterior arch of the atlas. This joint is a rotatory diarthrosis. Ligamenta. Each of the joints is furnished with a capsula articularis, whereby the articular cavity is circumscribed. In the case of the lateral articulations, each articular capsule presents a distinct band, named the accessory ligament, which is situated within the vertebral canal (Fig. 297), and passes downwards and medially from the lateral mass of the atlas to the superior aspect of the body of the epistropheus. The following additional ligaments constitute the leading bonds of union:Lig. Obturatorium Atlantoepistrophica Anterior. The anterior covering atlanto-epistropheal ligament (O.T. anterior atlo-axoid ligament) (Fig. 296) is a membranous structure which is thin laterally, but strong in the median plane, where it is thickened by a prolongation of the lig. longitudinale anterius. It extends from the anterior arch of the atlas to the front of the body of the epistropheus.