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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

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STERNAL ARTICULAR SURFACE

STERNO-HYOID

TUBEROSITAS CORACOIDEA.

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,

MAJOR

ELTO

RAP

FIG. 185. THE UPPER SURFACE OF THE RIGHT CLAVICLE WITH MUSCLE ATTACHMENTS.

ACROMIAL

ARTICULAR SURFACE

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

DELTOID TUBERCLE
TRAPEZOID LINE

ARTERIAL FORAMEN

-TUBEROSITAS CORACOIDEA

FIG. 186. THE RIGHT CLAVICLE SEEN FROM BELOW.

TUBEROSITAS COSTALIS

STERNAL
ARTICULAR
SURFACE

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

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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.

US

MAJOR

[graphic]

Conoid ligament

FIG. 187.-THE UNDER SURFACE OF THE RIGHT CLAVICLE WITH THE
ATTACHMENTS OF THE MUSCLES MAPPED OUT.

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

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 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.

Sternal epiphysis ossifies about 20th year; fuses about 25th year

FIG. 188.-OSSIFICATION OF THE CLAVICLE.

The Scapula.

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;

CLAVICULAR ARTICULAR SURFACE

SUPRA-SPINOUS FOSSA

Primary centres appear about
5th or 6th week of foetal life

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INFRA-SPINOUS FOSSA

ACROMIAL ANGLE

HEAD AND GLENOID CAVITY

NECK

GREAT SCAPULAR NOTCH

GROOVE FOR CIRCUMFLEX SCAPULAR ARTERY

INFERIOR ANGLE

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 scapula, rhomboideus minor, and rhomboideus major muscles.

It

The superior margin, which is thin and sharp, is the shortest of the three. 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,

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LONG HEAD

OF TRICEPS

GROOVE FOR CIRCUM

FLEX SCAPULAR
ARTERY

SCAPULAR SLIP OF LATISSIMUS
DORSI

FIG. 190.-THE DORSAL SURFACE OF THE RIGHT SCAPULA
WITH THE ATTACHMENTS OF THE MUSCLES MAPPED OUT.

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

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 col-
lum scapulæ (neck) is that
somewhat constricted part
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 at-
tached border of this crosses
the dorsal surface of the body obliquely in a direction.

CLAVICULAR ARTICULAR SURFACE
CORACOID PROCESS

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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 separ-
ated by the great scapular notch,
which here corresponds to the
dorsal aspect of the neck. Within
this notch the transverse scapu-
lar 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.
two fossæ are in communication

The

VERTERRAL MARGIN

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