Coracoid Process (Fig. 57).
This has already been described as arching forwards and a little outwards from the upper border of the bone over the tendon of Subscapularis : its tip is slightly depressed, showing a tendency to turn down in front of this muscle. We find, therefore, that its under surface is smooth and concave for the play of the tendon, from which it is separated by a subcoracoid bursa. Its other surfaces are roughened by the attachment of hgaments and tendons. Its upper surface ends behind in the rounded conoid tubercle from which the trapezoid ridge runs forwards along the outer part of the surface : in front of these is the shghtly marked roughness for Pectoralis minor, on the inner border and upper surface. The acromial branch of the acromio-thoracic axis crosses the process between the ligaments and the insertion of the tendon, under cover of the Deltoid. The costo-coracoid membrane, on which this artery hes at first, is attached to the inner part of the upper surface-between the hgaments and the insertion of Pectorahs minor. On the inner aspect are seen part of the attachment of this muscle and of the conoid ligament.
Looked at from the outer side a well-marked ridge is visible running forward toward the end of the process and lying immediately outside and below the trapezoid ridge. This outer edge shows two more or less definite markings, one near its posterior end (Fig. 57, 4) and a larger one near the front extremity, for the two main bands of the coraco-acromial ligament : between these the ridge affords attachment to the very weak central portion of the ligament, which is frequently absent.
Just below the level of the edge and the coraco-acromial ligament is the area of attachment of the coraco-humeral ligament, the apex of which extends far enough forward to he under cover of the anterior coraco-acromial band : the area widens behind and externally to enclose the supraglenoid tubercle (where the Biceps has origin for its long tendon), and becomes continuous on each side of this with the capsular marking.
Whatever may be the comparative value of the coracoid process, there is no doubt that it is one of the primary parts of the bone, and is concerned in the formation of a part of the glenoid fossa, as already shown in Fig. 58. The portion that aids in forming the cavity is ossified from a distinct centre, which, from its occurrence before the true epiphyses, probably is also of primary value. This and the main centre for the process join, and are afterwards furnished with small epiphyses on the prominent tubercles. The coracoid element (ventral) is separated from the scapular or dorsal element before fusion along the dark green line A in Fig. 57, which passes through the notch on the rim of the glenoid as well as on the front wall of the suprascapular notch.
The suprascapular notch is bounded in front by the base of the coracoid process, to which the bands of the suprascapular ligament are therefore attached : these bands may be two in number, an upper stronger part bridging the notch and attached to its margins and dorsal sides of these margins, and a lower weaker and more ventrally situated band. Under the lower band runs a vein passing up from the subscapular fossa to join the suprascapular vessels, while the nerve and one of the venae comites of the suprascapular artery pass between the two bands.
The omohyoid fascia is fastened to the conoid hgament and base of the coracoid process, passes from this on to the upper margin of the suprascapular hgament, and so gains the upper border of the bone behind the notch : here it encloses the Omohyoid muscle which arises from this part of the bone. Further back on the upper border, near the inner angle, fibres of Levator anguli scapulae are inserted, and their attachment is continued round the angle on to the vertebral border as far as the " base " of the spine. Here the lesser Rhomboid finds insertion, and below the level of the spine the Rhomboideus major insertion extends down on the border nearly to the lower angle.
The axillary border is a secondary border, not present in the young bone, in which this margin of the body is formed directly by the ventral bar : the adult border is slowly extended over this and partly round it as the Subscapularis grows, and thus accurately marks the extent of that muscle.
The bone is laid obliquely on the upper, outer, and back aspect of the thorax, so that its ventral surface really looks inwards as much as forwards, and in addition slightly downwards. Put in this position it forms with its front muscles the posterior (postero-external) wall of the axilla, being held out through its clavicular support and slung up by its muscular attachments to the skull and neck above it. When the clavicle is broken the bone and arm are no longer held out, but swing inwards on the muscular attachments, being displaced downwards and inwards through a small arc of a circle that has its centre at the muscular origins : this leads to depression of the outer fragment of the broken clavicle below the inner, which is thus prominent under the skin. True raising of the inner fragment occurs to a certain extent from unopposed action of Sterno-mastoid, but it is hmited by the costo-clavicular ligament, and a good deal of the upward displacement is only apparent, the result of the lowering of the outer piece. At the same time the inward and downward movement of the scapula can only take place on the dorsal aspect of the trunk, so that the outer fragment of the clavicle is also brought to a somewhat posterior plane, thus increasing the apparent prominence of the inner portion.
In the normal state the upper end of the vertebral border is nearer the middle line than the lower, being only about 2 to 3 inches from it. The lower angle varies, in this relation, with the position of the arm and the attitude of the shoulders. The scapula is separated from the ribs and intercostal muscles by Subscapularis and Serratus magnus, and in the ord'nary position of rest the angle lies over the seventh rib or space. The angle is not kept against the ches* by the Serratus magnus, but by the upper fibres of Latissimus dorsi (Fig. 39), which hold it hke a strap against the chest-wall : under this it can move round the wall for a distance of about an inch and a half as a result of the action of Serratus magnus or Trapezius.