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of fibro-cartilage which is interposed between it and the clavicular facet on the upper and lateral angle of the manubrium sterni. It is also supported by a small part of the medial end of the cartilage of the first rib. Its articular surface, usually broader from above downwards than from side to side, displays an antero-posterior convexity, whilst tending to be slightly concave in a vertical direction. The edge around the articular area, which serves for the attachment of the capsule of the
sterno-clavicular articulation, is sharp and well defined, except below, where it is rounded.
The body exhibits a double curve, being bent forwards in the medial twothirds of its extent, whilst in its lateral third it displays a backward curve. Of rounded or prismatic form towards its sternal end, it becomes compressed and flattened at its acromial extremity. It may be described as possessing two surfaces, a superior and an inferior, separated by anterior and posterior borders, which
are well defined towards the lateral extremity of the bone, but become wider and less well marked medially where they conform more to the cylindrical shape of the bone. The superior surface, which is smooth and subcutaneous throughout its whole length, is directed upwards and forwards. The anterior border,
which separates the superior from the inferior surface in front, is rough and tubercular towards its medial end for the attachment of the clavicular fibres of the pectoralis major, whilst laterally, where it becomes continuous with the anterior margin of the acromial end, it is better defined, and bears the imprint of the origin of the fibres of the deltoid muscle; here, not uncommonly, a projecting spur of bone, called the deltoid tubercle, may be seen. The posterior border is broad medially,
where it is lipped superiorly to furnish an attachment for the clavicular fibres of the sterno-mastoid muscle; behind and below this the sterno-hyoid and sterno-thyreoid muscles are attached to the bone. Laterally, the posterior border becomes more rounded, and is confluent with the posterior edge of the acromial end at a point where there is a marked outgrowth of bone from its inferior surface, the tuberositas coracoidea. Into the lateral third of this border are inserted the
upper and anterior fibres of the trapezius muscle. The inferior surface, inclined downwards and backwards, is marked close to the sternal end by
an irregular elongated
to this the shaft is
channelled by a groove
FIG. 187. THE UNDER SURFACE OF THE RIGHT CLAVICLE WITH THE
The morphology of the clavicle is of special interest. Its presence is associated with the freer use and greater range of movement of the fore-limb, such as are necessary for its employment for more specialised actions than those of mere progression. In consequence of these requirements, the limb, and with it the scapula, become further removed from the trunk, and so the support which the blade bone received through the union of its coracoid element with the sternum, as in birds and reptiles, and to some extent in the lowest mammals, is withdrawn. Some substitute, however, is necessary to meet the altered conditions, and in consequence a new element is introduced in the form of a clavicle. The origin of this bone appears to be intimately associated with the precoracoid element met with in amphibia or reptiles, but whereas the precoracoid is always laid down in cartilage, which, however, not infrequently disappears, the clavicle develops in the membrane overlying the precoracoid cartilage. In the course of its development it may become intimately associated with the remains of that cartilage. Thus, it is probable that the articular discs at the sterno-clavicular and acromio-clavicular joints, as well as the sternal articular end of the clavicle, represent persistent portions of the primitive cartilage, whilst it is possible that the supra-sternal ossicles occasionally present may be also derived from it. In this way, in its most specialised form, a secondary support is established between the sternum and scapula, which serves as a movable fulcrum, and greatly enhances the range of movement of the shoulder girdle.
Nutrient Foramina. The foramina for the larger nutrient vessels, offsets of the transverse scapular artery, of which there may be one or two directed laterally, are usually found about the middle of the posterior border, or, it may be, opening into the floor of the groove for the subclavius muscle.
Ossification. The clavicle in man is remarkable in commencing to ossify before any other bone in the body; this occurs as early as the fifth or sixth week of fœtal life. The shaft is ossified from two primitive centres (Mall). These are preceded by a curved rod of connective tissue on the interior of which are developed two masses of a peculiar precartilaginous nature, one, the sternal, placed medially, lies above and overlaps in front the acromial mass, which is placed laterally. In each of these near their approximated ends a centre of ossification appears. These, subsequent to the fusion of the
two independent precartilaginous masses, coalesce and form a bridge of bone uniting the two primary ossific centres. At a later stage cartilage cells appear in the medial
FIG. 188.-OSSIFICATION OF THE CLAVICLE.
extremity of the sternal precartilaginous mass and still later in the lateral end of
the acromial mass. By the growth and subsequent ossification of the cartilage so formed the clavicle increases in length (Fawcett).
A secondary centre appears at the sternal end about the age of twenty or later, and fusion rapidly occurring between it and the shaft, ossification is completed at the age of twenty-five or thereabouts.
The scapula, or shoulder blade, is of triangular shape and flattened form. It has two surfaces, costal or ventral, and dorsal. From the latter there springs a triangular process called the spine, which ends laterally in the acromion;
FIG. 189. THE DORSAL SURFACE OF THE RIGHT SCAPULA.
whilst from its superior margin there arises a beak-like projection called the coracoid process. The bone overlies the postero-lateral aspect of the thoracic framework, reaching from the second to the seventh rib.
The body of the bone, which is thin and translucent, except along its margins and where the spine springs from it, has three margins and three angles. Of these margins the vertebral (margo vertebralis) is the longest; it stretches from the medial angle above to the inferior angle below. Of curved or somewhat irregular outline, it affords a narrow surface for the insertion of the levator scapulæ, rhomboideus minor, and rhomboideus major muscles.
The superior margin, which is thin and sharp, is the shortest of the three. It runs from the medial angle towards the root of the coracoid process, before reaching which, however, it is interrupted by the scapular notch, which lies very close to the medial side of the base of that process. This notch, which is converted into a foramen by a ligament, or occasionally by a spicule of bone, transmits the suprascapular nerve, whilst the transverse scapular artery runs above it. Attached to the superior margin, close to the notch, is the posterior belly of the omo-hyoid. The axillary margin, so called from its relation to the hollow of the axilla (armpit), is much stouter than either of the others; it extends from the lateral angle above to the inferior angle below. The upper inch or so of this border, which lies immediately below the glenoid articular cavity, is rough and tubercular (tuberositas infraglenoidalis), and affords attachment to the long head of the triceps. Below this it is usually crossed by a groove which marks the position of the circumflex scapular artery.
The medial angle is sharp and more or less rectangular; the inferior angle is blunter and more acute; whilst the lateral angle corresponds to that part of the bone which is sometimes called the head,
GROOVE FOR CIRCUM
FIG. 190.-THE DORSAL SURFACE OF THE RIGHT SCAPULA
and which supports the glenoid cavity and the coracoid process.
The glenoid cavity is a piriform articular area, slightly concave from above downwards and from side to side; its border is but slightly raised above the general surface and affords attachment in the recent condition to the labrum glenoidale, which helps to deepen the socket in which the head of the humerus rests. Below, the margin of the glenoid cavity is confluent with the infra-glenoidal tuberosity, whilst, above, it blends with a tubercle (tuberositas supraglenoidalis), to which the long head of the biceps muscle is attached. Springing from the upper part of the head, in line with the superior margin, is the processus coracoideus (coracoid process). The base of this is limited laterally by the glenoid edge, whilst medially it is separated from the superior margin by the scapular notch. Rising upwards for a short space, it bends on itself at nearly a right angle, and ends in a process which is directed laterally and slightly forwards, overhanging the glenoid cavity above and in front. Compressed from above downwards, it has attached to its upper surface near its angle the conoid ligament, lateral to which there is a rough area for the trapezoid ligament. Attached to its dorsal border is the coraco