General features.-The clavicle, though a long bone, differs from the other long bones in the body in not possessing a medullary cavity. It lies almost horizontally in the body at the root of the neck and is subcutaneous throughout its whole extent. Its most important functions are : (1) to act as a prop which braces back the shoulder and enables the limb to swing clear of the trunk : and (2) to transmit a part of the weight of the limb to the axial skeleton, in this way diminishing the muscular effort required for that purpose. The lateral or acromial end of the bone is flattened and articulates with the medial side of the acromion, whereas the medial or sternal end is enlarged and articulates with the clavicular notch of the manubrium sterni. The shaft is gently curved and is shaped somewhat like the italic letter f, being convex forwards in its medial portion and concave forwards in its lateral portion. The inferior aspect of the intermediate third is grooved in its long axis. The student is now in a position to refer a given clavicle to its correct side of the body.
For purposes of description it is convenient to divide the shaft into its lateral one-third, which is flattened, and its medial two-thirds, which are cylindrical or prisinoid in form.
The medial two-thirds of the shaft of the clavicle usually possess four surfaces, but, at the sternal end of the bone, the inferior surface is often reduced to a mere ridge. The anterior surface is roughened over most of its extent but it is smooth and rounded at its lateral end, where it forms the upper boundary of the infraclavicular fossa. The upper surface, also, is roughened in its medial part and smooth at its lateral end. The posterior surface is smooth and featureless. The inferior surface is marked, near the sternal end, by a roughened impression, which is often depressed below the surface. This gives attachment to the costoclavicular ligament, which connects the clavicle to the upper surface of the first rib and helps to limit its range of movement. The lateral half of this surface shows a groove in the long axis of the bone.
The flattened acromial end of the clavicle presents a small oval articular facet, which articulates with the medial aspect of the acromion at the acromioclavicular joint. The facet is directed laterally and slightly downwards. The sternal end of the clavicle is directed medially, and a little downwards and forwards, to articulate with the clavicular notch of the manubrium sterni. The sternal surface is quadrangular (sometimes triangular) in form, and its uppermost part is slightly roughened for ligamentous attachments. Elsewhere, in a normal bone, the surface is smooth and articular and it is carried round on to the inferior surface for a short distance, where it articulates with the first costal cartilage. The sternal end of the clavicle projects above the manubrium stern] and can be felt without difficulty in the lateral wall of the suprasternal fossa.
Particular features.–The lateral one-third of the shaft gives attachment to the deltoid muscle by its anterior border, and to the trapezius muscle by its posterior border. Both muscles tend to encroach on the upper surface. The coracoclavicular ligament, attached to the conoid tubercle and the trapezoid ridge (fig. 405), transmits a large part of the weight of the upper limb to the clavicle. This weight is counteracted by the toms of the trapezius muscle, which supports the lateral part of the bone. From the conoid tubercle the weight is transmitted through the medial two-thirds of the shaft to reach the axial skeleton. Fracture of the clavicle medial to the conoid tubercle interrupts the line of weight transmission, so that practically the whole weight of the limb has to be supported by the trapezius. The muscle is unable to meet the demand and the limb therefore drops on the affected side.
The medial two-thirds give attachment anteriorly to the clavicular head of the pectoralis major muscle, and, as a rule, the area is clearly indicated on the bone. The clavicular bead of the sternomastoid arises from the media half of the upper surface, but the marking on the bone is not very conspicuous. The smooth posterior surface is devoid of muscular attachments except at its lower part immediately adjoining the sternal end. where the lateral fibers of the sternohyoid arise. Medially, this surface is related to the terminal parts of the internal jugular, from which it is separated by the sternohyoid muscle, and the subclavian veins and the commencement of the innominate vein. More laterally, it arches above and in front of the trunks of the brachial plexus and the third part of the subclavian artery. The suprascapular vessels are related to the upper part of this surface. The inferior surface gives insertion to the subclavius muscle along the subclavian groove (fig. 405), and the edges of the groove give attachment to the clavipectoral fascia, which encloses the muscle. The posterior lip of the groove runs into the conoid tubercle and carries the fascia into continuity with the conoid ligament. A nutrient foramen is found in the lateral end of the groove, running in a lateral direction. The nutrient artery concerned is derived from the suprascapular (transverse scapular) artery. The impression for the costoclavicular ligament, which is very variable in its character, is separated from the sternal end by a short interval.
The roughened, upper part of the sternal end provides attachment for the interclavicular ligament, the articular capsule and the articular disc of the sternoclavicular joint. The sternal surface, denuded of its articular cartilage, is rarely smooth and is usually irregular and somewhat pitted.
In the female the clavicle is shorter,* thinner, less curved and smoother than in the male. In the female the acromial end is a little below the level of the sternal end ; in the male it is on a level with, or slightly higher than, the sternal end. In persons who perform hard manual labor the clavicle is thicker and more curved, and its ridges for muscular attachment are better marked.
* F. G. Parsom (Journal of Anatomy and Physiology, vol. lI.) gives the following as the average lengths of the clavicle in the male and female ; male, left, 154 mm., right, 152 mm. ; female, left, 139 mm, right, 138 mm.
Structure.-The clavicle consists of spongy substanoe, enveloped by a layer of compact bone which is much thicker in the intermediate part than at fihe ends of the bone.
Ossification.-The. clavicle begins to ossify before any other bone in the body, and is ossified from three centers. The body of the bone is ossified in membrane from two primary centers,* a medial and a lateral; which appear between the fifth and sixth weeks of fetal life, and .fuse about the forty-fifth day, a secondary center for the sternal end appears about the eighteenth or twentieth year, and unites with the body of the bone about the twenty-fifth year. A secondary center of ossification sometimes develops in the cartilage at the acromial end at about eighteen to twenty. The. epiphysis so formed is always small and rudimentary and rapidly joins the rest of the bone.
In a 14 mm. embryo the future clavicle is represented by a band of mesenchyme which extends from the acromion of the scapula to the tip of the first rib, and is continuous with the rudiment of the sternum. In this band a medial and a lateral mass of ‘precartilage’ is developed, and in the mesenchyme intervening between them the two centers for the body of the bone appear and soon fuse with each other. The sternal and acromial parts of the precartilaginous masses are converted into cartilage, and into this the ossification of the body of the bone extends.
The primitive reptilian shoulder girdle comprises a dorsal element, the scapula, and two ventral elements, of which the anterior (head-ward) is the precoracoid and the posterior (caudal) is the coracoid. The primitive girdle of the hand limb also possesses three elements, of which the ilium is homologous with the scapula, the pubis with the precoracoid and the ischium with the coracoid. The clavicle, which is a membrane bone and therefore morphologically distinct from the others, is an additional element in the shoulder girdle but is not represented in the pelvic girdle. It is doubtful whether any trace of the precoracoid persists in the human skeleton, although the presence of two primary centers of ossification for the clavicle is regarded by many authorities as an indication that the human clavicle corresponds both to the precoracoid and to the clavicle in the reptilian shoulder-girdle.
Applied Anatomy.-The clavicle is very frequently fractured, since it is much exposed to violence, and is the only bony connection between the upper limb and the trunk acting as a buttress to keep the point of the shoulder away from the thorax. It is moreover, slender, and is very superficial. The most common cause is indirect violence, as the result of force applied to the hand or shoulder, and the bone then gives way at the junction of its lateral with its intermediate third, that is to say, at the junction of the two curves, for this is its weakest part. The deformity is mainly due to the weight of the arm acting upon the fragment when the buttress-like action of the bone is gone, assisted by the muscles which pass from the thorax to the upper extremity. The medial fragment, as a rule, is little displaced.
Great deformity of the clavicle may be met with in rickets, the natural curvatures of the bone being exaggerated until it takes an ï¿½Sï¿½ shape, and ‘green-stick’ fracture is not uncommonly seen associated therewith.
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