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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
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).
Primary centre appears about 2nd m. fœtal life.
Acromial centres appear 15-16 yrs. ; fuse about 25 yrs.
Secondary centre for
Scapula at end of First Year.
Appears about 16-17 yrs.; fuses about 20 yrs.
Appears about 16 or 17 yrs. ; fuses 18-20 yrs.
Appears 16-17 yrs.;
Scapula about the Age of Puberty.
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.
The proximal extremity is the thickest and stoutest part of the bone. The caput humeri (head), which forms about one-third of a spheroid and is covered with
articular cartilage, is directed proximally, medially, and slightly dorsally, and rests in the glenoid cavity of the scapula; the convexity of its surface is most pronounced in its posterior half. Separating the head from the tubercles laterally is a shallow groove, which fades away on the surface of the bone which supports the articular part inferiorly. This is named the collum anatomicum (anatomical neck) and serves for the attachment of the capsule of the shoulder-joint. The
articular edge of the groove opposite the lesser tubercle is usually notched for the attachment of the superior gleno-humeral ligament. The tuberculum majus (greater
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
FIG. 184. THE RIGHT CLAVICLE SEEN FROM ABOVE.
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,
ACROMIAL ARTICULAR SURFACE
FIG. 185. THE UPPER SURFACE OF THE RIGHT CLAVICLE
FIG. 186.-THE RIGHT CLAVICLE SEEN FROM BELOW.
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 impression (tuberositas costalis), often deeply pitted, for the attachment of the costoclavicular ligament, which unites it to the cartilage of the first rib. Lateral to this the shaft is channelled by a groove which terminates close to the coracoid impression; into this groove the subclavius muscle is inserted.
The acromial end of the bone is flattened and compressed from above downwards, and expanded from before backwards; its anterior edge is sharp and well defined, and gives attachment to the deltoid muscle, which also spreads over part of its upper surface. Its posterior margin is rougher and more tubercular, and provides a surface for the insertion of the trapezius. The area of the superior surface between these two muscular attachments is smooth and subcutaneous. The lateral edge of this forward-turned part of the bone is provided with an oval facet (facies articularis acromialis) for articulation with the acromion of the scapula; the margins around this articular area serve for the attachment of the capsule of the joint. The inferior surface of the acromial end of the bone is traversed obliquely from behind forwards and laterally by a rough ridge or line called the trapezoid ridge. The posterior extremity of this ridge, as it abuts on the posterior border of the bone, forms a prominent process, the tuberositas coracoidea; to these, respectively, are attached the trapezoid and conoid portions of the coraco-clavicular ligament.
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 foetal 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