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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
acromial ligament, whilst at ACROMION its extremity and towards the front of its ventral border, is the combined origin of the biceps and coracobrachialis, together with the insertion of the pectoralis minor. The collum scapulæ (neck) is that somewhat constricted part GLENOID of the bone which supports the head; it corresponds in front and behind to a line drawn from the scapular notch to the infra-glenoidal tuberosity.
The body of the bone has two surfaces, a dorsal (facies dorsalis) and a costal (facies costalis). The former is divided into two fosse by an outstanding process of triangular form, called the spina scapulæ. The attached border of this crosses
CLAVICULAR ARTICULAR SURFACE
FIG. 191.-THE RIGHT SCAPULA
laterally and slightly upwards, extending from the vertebral border, near the lower limit of its upper fourth, towards the centre of the posterior glenoid edge, from which, however, it is separated by the great scapular notch, which here corresponds to the dorsal aspect of the neck. Within this notch the transverse scapular vessels and the supra-scapular nerve pass to the infra-spinous fossa. The surfaces of the spine, which are directed upwards and downwards, are concave, the upper entering into the formation of the supra-spinous fossa, which lies above it, the lower forming the upper wall of the infra-spinous fossa, which lies below it. The two fossæ are in communication
FIG. 192. COSTAL SURFACE OF THE RIGHT SCAPULA WITH THE ATTACHMENTS OF MUSCLES MAPPED OUT.
with each other round the free lateral concave border of the spine, where that curves over the great scapular notch. The dorsal free border of the spine is subcutaneous throughout its entire length. Its upper and lower edges are strongly lipped, and serve the superior, for the insertion of the trapezius; the inferior, for the origin of the deltoid. The intervening surface varies in width-broad and triangular where it becomes confluent with the vertebral border, it displays a smooth surface, over which the tendinous fibres of the trapezius play; narrowing rapidly, it forms a surface of varying width which blends laterally with a flattened process, the two forming a compressed plate of bone which arches across the great scapular notch above and behind, and then curves, upwards, forwards, and laterally to overhang the glenoid cavity. The medial border of this process is continuous with the upper margin of the spine, and is gently curved. The lateral border, more curved than the medial, with which it is united in front, is confluent with the inferior edge of the spine, with which it forms an abrupt bend, termed the acromial angle. The bone included between these two borders is called the acromion. Of compressed form, it much resembles the acromial end of the clavicle, with which it articulates by means of a surface (facies articularis acromii) which is placed on its medial border near its anterior extremity. The superior surface of the acromion, which is broad and expanded, is subcutaneous, and is directed upwards and dorsally, and in the normal position of the bone laterally as well. Its medial edge, where not in contact with the clavicle, has attached to it the fibres of the trapezius, whilst its lateral margin affords origin to the central part of the deltoid. At its anterior extremity it is connected with the coracoid process by means of the coracoacromial ligament. Its inferior surface is smooth and overhangs the shoulder-joint. The supra-spinous fossa, of much less extent than the infra-spinous, is placed above the spine, the upper surface of which assists in forming its curved floor; in it is lodged the supraspinatus muscle. The scapular notch opens into it above, whilst below and laterally it communicates with the infra-spinous fossa by the great scapular notch, through which the transverse scapular artery and suprascapular nerve pass to reach the infra-spinous fossa.
The infra-spinous fossa, overhung by the spine above, is of triangular form. The axillary margin of the bone limits it in front, whilst the vertebral margin bounds it behind; the greater part of this surface affords origin to the infraspinatus muscle, excepting a well-defined area which skirts the axillary margin and inferior angle of the bone, and which affords an attachment to the fibres of origin of the teres minor. This muscle extends along the dorsal surface of the axillary margin in its superior two-thirds, reaching nearly as high as the glenoid edge; whilst a crescentic surface, which occupies the inferior third of the axillary border and curves backward round the dorsal aspect of the inferior angle, furnishes an origin for the teres major muscle. Here also, near the inferior angle, are occasionally attached some of the fibres of the latissimus dorsi muscle.
The facies costalis (costal aspect) of the body is hollow from above downwards and from side to side, the greatest depth being in correspondence with the spring of the spine from the dorsal surface. Its medial boundary, which is formed by the anterior lipped edge of the vertebral margin, affords attachment to the fibres of insertion of the serratus anterior along the greater part of its extent. The area of insertion of this muscle is, however, considerably increased over the ventral aspects of the medial and inferior angles respectively. Running down from the head and neck above to the inferior angle below, there is a stout rounded ridge of bone, which imparts a fulness to the costal aspect of the axillary margin and increases the depth of the costal hollow; to this, as well as to the floor of the fossa, the subscapularis muscle is attached. The tendinous intersections of this muscle leave their imprint on this surface of the bone in a series of three or four rough lines which converge towards the neck.
The scapula of man is characterised by the greater proportionate length of its base or vertebral margin as compared with lower forms. This proportion is expressed by what is termed the scapular index (Appendix D). The greater size of the acromion is also a distinctive feature. The double ossification of the coracoid occurs only in mammals. It is probable that the centre for the upper and anterior part of the
coracoid process represents the epicoracoid or precoracoid of lower forms, whilst the subcoracoid centre (metacoracoid) which assists in the formation of the glenoid cavity is the reduced and vestigial remains of the stout coracoid element met with in Ornithorhynchus, which articulates with the sternum.
Nutrient Foramina. Foramina for the passage of nutrient vessels are seen in different parts of the bone; the most constant in position is one which opens into the infra-spinous fossa, about an inch or so from the scapular notch. Others are met with on the upper and under surfaces of the spine, on the costal aspect near its deepest part, and also around the glenoid margin.
Connexions.-The scapula is not directly connected with the trunk, but articulates with the lateral end of the clavicle, in union with which it forms the shoulder girdle, supporting the humerus on its glenoid surface. Placed on the upper and back part of the thorax, it covers the ribs from the second to the seventh inclusive. Possessed of a wide range of movement, it alters its position according to the attitude of the limb, rising or falling, being drawn medially or laterally, or being rotated upon itself according as the arm is moved in various directions. These changes in position can easily be determined by recognising the altered relations of the subcutaneous and bony prominences, more especially the former, which include the spine, the acromion, and the inferior half of the vertebral border.
Ossification.-Ossification begins in the body of the cartilaginous scapula about the end of the second month of foetal life. At birth the head, neck, body, spine, and base of the coracoid process are well defined; the vertebral margin, inferior angle, glenoid cavity, acromion, and coracoid process, are still cartilaginous. The centre for the upper and anterior part of the coracoid appears in the first year, and fusion, along an oblique line leading from the upper edge of the glenoid cavity to the conoid tubercle, is complete about the fifteenth year. A separate centre (subcoracoid), which ultimately includes the superior part
Scapula at end of First Year.
Scapula about the Age of Puberty.
of the glenoid cavity and lateral part of the coracoid process, makes its appearance about the tenth year, and fuses with the surrounding bone about sixteen or seventeen. Up till the age of puberty the acromion remains cartilaginous; centres, two or more in number, then make their appearance, which coalesce and ultimately unite with the spine about the twenty-fifth year. Failure of union may, however, persist throughout life (see Appendix B-Variations).
Ossification commences in the cartilage in the inferior angle about puberty, and independently and a little later, along the vertebral margin, fusion with the body occurring at from twenty to twenty-five years.
Small scale-like epiphyses make their appearance on the superior surface and at the extremity of the coracoid, and are completed about the twentieth year. A thin epiphysial plate develops over the inferior part of the glenoid cavity about sixteen or seventeen, fusion being complete about eighteen or twenty years of age.
The humerus, or bone of the arm, articulates proximally with the scapula and distally with the bones of the forearm, namely, the radius and ulna. Its proximal end comprises the head and greater and lesser tubercles; its body, which is longer than any of the other bones of the upper extremity, is cylindrical proximally and flattened distally. At the distal extremity, which is expanded to form the epicondyles on either side, it supports the trochlear and capitular articular surfaces for the ulna and radius respectively.