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extensions to the adjoining rib and its costo-transverse articular capsule. Others proceed almost vertically upwards to the adjoining transverse process, while those which ascend from the upper surface of the tubercle pass obliquely upwards and inwards to reach the postero-inferior aspect of the adjoining transverse process. Some posterior fibres connected with the transverse process at its junction with the lamina are called the posterior costo-transverse ligament.

Lig. Tuberculi Costæ.—The ligament of the tubercle of the rib is a band of transverse fibres applied to the postero-lateral aspect of the capsule. By one end these fibres are attached to the tip of the transverse process behind its articular facet, and by the other to the external rough surface of the tubercle of the rib.

Lig. Colli Costæ.— The ligament of the neck of the rib (O.T. middle costotransverse ligament) consists of short fibres which stretch from the posterior aspect of the neck of the rib, backwards and medially, to the anterior aspect of the transverse process, but, in addition, a proportion of the fibres passes to the posterior aspect of the inferior articular process of the upper of the two vertebræ with which the head of the rib articulates.

The following exceptions to the general plan of rib-articulation indicated above must be noted :1. There is no articulation between the eleventh and twelfth ribs and the

transverse processes of the corresponding vertebra. 2. The anterior costo-transverse ligament is wanting from the first rib, and is

either rudimentary or wanting in the case of the twelfth rib. 3. The lig. colli costa is rudimentary in the eleventh and twelfth ribs.

The ligamentum lumbocostale extends from the superior surface of the base of the transverse process of the first lumbar vertebra to the inferior surface of the neck of the twelfth rib, as well as to the inferior surface of the transverse process of the twelfth thoracic vertebra.

ARTICULATIONES COSTOCHONDRALES. Each rib possesses an unossified portion, termed its costal cartilage. As age advances, this cartilage may undergo a certain amount of superficial ossification, but it never becomes entirely transformed. The line of demarcation between bone and cartilage is clear and abrupt, and usually the bone forms an oval cup, in which the end of the cartilage is retained by means of the continuity which exists between the periosteum and the perichondrium. There is no articulation in the proper sense between the rib and its cartilage, although a synovial cavity has occasionally been found between the first rib and its cartilage.

ARTICULATIONES INTERCHONDRALES. Interchondral joints are arthrodial diarthroses, and they are found between adjoining margins of certain of the costal cartilages, viz., from the fifth to the eighth or ninth. The cartilages which thus articulate develop flattened, somewhat conical, prolongations of their substance, and thereby the intercostal spaces are interrupted where these flat articular facets abut against each other. Each joint is closed by a surrounding articular capsule, the superficial and thoracic aspects of which are specially strengthened by external and internal interchondral ligaments. These bands extend obliquely between adjacent cartilages.

A stratum synoviale lines each joint capsule.

ARTICULATIONES STERNOCOSTALES. The upper seven pairs of costal cartilages, as a rule, extend to the lateral margins of the sternum to form sterno-costal joints. Of these, the first pair is implanted directly upon the manubrium sterni. The ossific process ends abruptly in connexion with the rib, and also ceases as suddenly in connexion with the sternum, and hence the cartilage does not normally present an articulation at either end. From the second to the seventh pairs of ribs inclusive, the sterno-costal joints are constructed upon the type of arthrodial diarthroses, although, in the case of the sixth and seventh cartilages, the joint cavity is always small, and is frequently obliterated.

The sternal end of each of these costal cartilages presents a slight antero-posterior ridge which fits into a shallow V-shaped depression upon the lateral margin of the sternum. With the exception of the sixth cartilage, they articulate opposite the lines of union between the primary segments of the sternum; the sixth articulates upon the side of the lowest segment of the body of the sternum.

Each joint is enclosed by a capsula articularis, the fibrous stratum of which is attached to the adjacent borders of the articulating elements. Specially strong fibres distinguish the superficial and deep aspects of the capsule.

The lig. sternocostale radiatum (O.T. anterior costo-sternal ligament) (Fig. 301) is composed of strong fibres which radiate from the anterior surface of the costal cartilage, near its sternal end, to the front of the sternum. The ligaments of opposite sides interlace with each other, and so cover the front of the sternum with a felted membrane—the membrana sterni.

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The lig. sternocostale posterius (posterior costo-sternal ligament)—also a part of the capsule—has attachments similar to the foregoing, but the arrangement of its fibres is not so powerful.

The ligamentum costoxiphoideum passes from the front of the upper part of the xiphoid process, obliquely upwards and laterally to the front of the seventh, and sometimes to the front of the sixth costal cartilage.

Within the capsules of these joints ligamenta sternocostalia interarticularia (interarticular ligaments) (Fig. 301) may be found. Their disposition is somewhat uncertain, for whereas, in the case of the second pair of cartilages, they invariably divide the joint cavity into two distinct compartments-an upper and a lower - such an arrangement is very uncertain in the other joints, and they occasionally, especially in the cases of the sixth and seventh cartilages, entirely obliterate the joint cavity. These ligaments extend horizontally between the ends of the costal cartilages and the side of the sternum.

The stratum synoviale is found wherever a joint cavity is developed, and therefore there may be one or two synovial strata, according to the presence or absence of a proper interarticular ligament. When the joint cavity is obliterated by the fibrous structure which represents the interarticular ligament, a synovial stratum is also absent.

ARTICULATIONES STERNI.

Primarily the sternum consists of an elongated plate of hyaline cartilage, which becomes subdivided into segments by the process of ossification.

The four segments of which the body of the sternum is originally composed unite with each other after the manner of typical synchondroses.

Similarly the xiphoid process and the body ultimately become united. It is not usual to find the joint between the manubrium and the body obliterated by the ossification of the two bony segments. Even in advanced life it remains open, and the joint, which is named the synchondrosis sternalis, partakes of the nature of an amphiarthrosis (Fig. 301), although a joint cavity is not found under any circumstances in the plate of fibro-cartilage which intervenes between the manubrium and the body of the sternum.

The membrana sterni, to which reference has already been made, assists in strengthening the union between the different segments of the sternum.

Movements of the Ribs and Sternum.— These movements may be considered either independently of, or as associated with, respiration.

In the former condition the ribs move in connexion with flexion and extension of the vertebral column, being more or less depressed and approximated in the former, and elevated or pulled apart in the latter case. Considered in connexion with respiration, it is necessary to observe that, to all intents and purposes, the vertebral column and the sternum are rigid structures. Next, we must remember that the heads of all the ribs occupy fixed positions, and similarly the anterior ends of seven pairs of cartilages are fixed to the lateral margins of the sternum. The ribs thus form arches, presenting a large amount of obliquity from behind forwards. Therefore, during inspiration, when the rib is elevated, the arch becomes more horizontal, and the transverse diameter of the chest is increased. At the same time, the anterior ends of the sternal ribs tend to thrust the sternum forwards and upwards; but the nature of the attachment of the first pair of ribs to the sternum, as well as the attachment of the diaphragm to the xiphoid process, prevents this movement from becoming excessive, and hence the sternum becomes a line of resistance to the forward thrust of the ribs. As a consequence, the ribs rotate upon themselves about an oblique axis which passes downwards, laterally, and posteriorly through the capitular joint and the neck of the rib anterior to the costo-transverse joint.

In this way increase, both of the antero-posterior and transverse diameters of the thorax, is provided for, although the amount of increase is not equally pronounced in all planes. Thus at the level of the first rib very little eversion is possible, because the axis of rotation is nearly transverse, and therefore any increase in the transverse or antero-posterior thoracic diameters at this level may be disregarded, although a certain amount of elevation of the manubrium sterni and anterior end of the first rib is evident.

Below the level of the sixth rib elevation and rotation of the rib during inspiration are usually said to be complicated by a certain amount of backward movement, due to the character of the costo-transverse joint, until, in the case of the last two ribs, which are destitute of costotransverse joints, a movement backwards is almost entirely substituted for elevation. It is probable, however, that the movements of the asternal ribs exactly correspond to those of the sternal series, and that by the contraction of the costal digitations of the diaphragm the anterior ends of the false ribs are provided with fixed positions comparable to those supplied by the sternum to the ribs of the sternal series.

We may therefore say that during inspiration the ribs move upwards and laterally between their fixed ends, while as a whole the rib rotates, and its anterior end is thrust slightly forwards.

During expiration these movements are simply reversed.

THE ARTICULATIONS OF THE SUPERIOR EXTREMITY.

The bony arch formed by the clavicle and scapula articulates directly with the axial skeleton only at one point, viz., the sterno-clavicular joint.

ARTICULATIONS OF THE CLAVICLE.

ARTICULATIO STERNOCLAVICULARIS.

The sterno-clavicular joint is an example of an arthrodial diarthrosis. The articular surfaces concerned in its formation present the following appearances :

1. The sternal end of the clavicle is somewhat triangular in outline, having its most prominent angle directed inferiorly and posteriorly. The anterior and posterior sides of the triangle are slightly roughened for the attachment of ligaments, while the base or inferior side is smooth and rounded, owing to the prolongation of the articular surface to the inferior aspect of the bone. In the antero-posterior direction the articular surface tends to be concave, while vertically it is slightly convex.

2. An articular facet, situated on the superior lateral angle of the manubrium sterni, but in a plane slightly behind the supra-sternal notch, articulates with he clavicle. This facet is considerably smaller than the clavicular facet with which it articulates.

3. The superior surface of the first costal cartilage close to the sternum also participates to a small extent in the articulation.

It should be noted that the articular surfaces of the clavicle and sternum are covered mainly by fibro-cartilage.

A capsula articularis is well marked on all sides except inferiorly, where it is very thin. The epiphyseal line of the clavicle is intra-capsular.

Lig. Sternoclaviculare Anterius. --- The anterior sterno-clavicular ligament (Fig. 301) forms part of the fibrous stratum of the articular capsule, and consists of short fibres which extend obliquely inferiorly and medially from the anterior aspect of the sternal end of the clavicle to the adjoining anterior surface of the sternum and the anterior border of the first costal cartilage.

Lig. Sternoclaviculare Posterius. — The posterior sterno-clavicular ligament also forms part of the fibrous stratum of the capsule, and consists of similarly disposed, but not so strong as the anterior ligament, oblique fibres situated on the posterior aspect of the articulation.

Discus Articularis.--A fibro-cartilaginous articular disc (Fig. 301) divides the joint cavity into two compartments. It is nearly circular in shape, and adapts itself to the articular surfaces between which it lies. It is thickest at the circumference and thinnest at the centre, where it occasionally presents a perforation, thereby permitting the two synovial cavities to inter-communicate. By its circumference it is in contact with, and adherent to, the surrounding capsule, but its superior margin is attached to the apex of the articular surface of the clavicle, while by its inferior margin it is fixed to the sternal end of the first costal cartilage.

Two accessory ligaments are associated with this joint, viz., the interclavicular and the costo-clavicular.

Lig. Interclaviculare.—The interclavicular ligament (Fig. 301) is a structure of considerable strength, forming a broad band of fibrous tissue which is attached to the superior rounded angle or apex of the sternal end of the clavicle as well as to the adjacent margins of the articular surface. Its fibres pass across the interclavicular notch to become attached to corresponding parts of the opposite clavicle, but in their course they dip down into the supra-sternal notch, in which many of them are fixed to the sternum. In this way their presence neither bridges nor obliterates the notch between the two clavicles, and the ligament really becomes a superior sternoclavicular ligament for each joint.

Lig. Costoclaviculare.—The costo-clavicular ligament (Fig. 301) consists of short, strong fibres which are attached inferiorly to the superior surface of the first costal cartilage. They pass obliquely upwards, laterally and posteriorly, to a rough impression situated on the inferior aspect of the sternal end of the clavicle, and are distinct from the articular capsule. Occasionally a bursa is found in the interior of this ligament.

As a rule there is a synovial stratum lining each of the two joint cavities (Fig. 301), separated from each other by the articular disc. Sometimes, however, the two membranes establish continuity through a perforation in the disc.

ARTICULATIO ACROMIOCLAVICULARIS. The acromio-clavicular joint is another instance of an arthrodial diarthrosis. It is situated between the acromial end of the clavicle and the medial aspect of the acromion. Each articular surface is an oval, flattened facet, covered with fibro-cartilage.

The ligaments which surround this small joint form a complete articular capsule, of which the superior and inferior parts are specially strong, and are therefore named the superior and inferior acromio-clavicular ligaments (Fig. 303). These consist of short fibres passing between the adjacent rough margins of the two bones in the positions indicated by their names.

An articular disc, which is nearly always incomplete, and may occasionally be wanting, is usually found within the joint cavity, where it lies obliquely, with its superior margin farther from the median plane than its inferior margin, and having its borders attached to the surrounding capsule. Frequently the disc is wedgeshaped, with its base directed upwards and its apex free.

A synovial stratum is found forming either a single or a double sac, according to the condition of the disc. Complete division of the joint cavity, however, is rare.

Ligamentum Coracoclaviculare.— Accessory to this articulation there is the strong coraco-clavicular ligament which binds the acromial end of the clavicle to the coracoid process of the scapula. It is readily divisible into two parts, viz., lig. conoideum and the lig, trapezoideum.

The conoid ligament (Fig. 303) is situated medial to and slightly posterior to the trapezoid. It is narrow and pointed at its inferior end, by which it is attached to the superior aspect of the coracoid process, in close proximity to the scapular

notch. Its superior end widens out in the manner expressed by its name, and is ? attached to the coracoid tuberosity of the clavicle.

The trapezoid ligament (Fig. 303) is attached inferiorly to the superior surface of the posterior half of the coracoid process, lateral and anterior to the attachment of the conoid ligament. Superiorly it is attached to the ridge on the inferior surface of the acromial end of the clavicle. Its lateral and medial borders are free.

Its anterior surface is principally directed upwards, and its posterior surface, to a similar extent, looks downwards.

A mucous or synovial bursa usually occupies the re-entrant angle between these two ligaments.

Movements at the Clavicular Joints. The movements of the medial end of the clavicle at the sterno-clavicular joint are limited in their range, owing to the tension of the ligaments. When the shoulder is raised or depressed the acromial end of the clavicle moves upwards and downwards, whilst its sternal end glides upon the surface of the articular disc; when, on the other hand, the shoulder is carried forwards or backwards, the sternal end of the clavicle along with the articular disc moves upon the sternal facet. In addition to these movements of elevation, depression, forward movement and backward movement of the clavicle, there is also allowed at the sterno-clavicular joint a certain amount of circumduction of the clavicle.

The part which is played by certain of the ligaments in restraining movement requires careful consideration. The costo-clavicular ligament checks excessive elevation of the clavicle, and restrains within certain limits both backward and forward movement of the clavicle. When the clavicle is depressed, as in cases where a heavy weight, such as a bucket of water, is carried in the hand, it receives support by resting upon the first rib, and the tendency for the medial end of the bone to start up out of its sternal socket is obviated by the tension of the articular disc, the interclavicular ligament, and the anterior and posterior sterno-clavicular ligaments.

The articular disc not only acts as a cushion which lessens the shock of blows received upon the shoulder, but it also acts as a most important bond of union, and prevents the medial end of the clavicle from being driven upwards upon the top of the sternum when force is applied to its lateral end.

The movements at the acromio-clavicular joint are of such a kind as to allow the inferior angle, and to some extent the vertebral border of the scapula, to remain more or less closely applied to the chest-wall during the various movements of the shoulder. The strong connexion between the coracoid process and the acromial end of the clavicle, by means of the conoid and trapezoid ligaments, renders it necessary that the scapula should follow the clavicle in its various excursions. The presence of the acromio-clavicular joint, however, enables the scapula to change its position somewhat with reference to the clavicle as the shoulder is moved. Thus, when the shoulder is raised and depressed, a marked difference takes place in the angle between the two bones ; again, when the shoulder is thrown forwards or backwards, these movements can be performed without altering in a material degree the direction of the glenoid cavity of the scapula, or in other words, the socket of the shoulder-joint.

The conoid and trapezoid ligaments set a limit upon the movements of the scapula at the acromio-clavicular joint. They both, but more particularly the trapezoid ligament, prevent the acromion from being carried medially below the lateral end of the clavicle when blows fall upon the lateral aspect of the shoulder.

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