The bones entering into the construction of, and forming the basis on which the shoulder is constructed are the clavicle and scapula, forming the shoulder-girdle, and the humerus.

The clavicle is a comparatively long and slender bone that acts as a prop to keep the point of the shoulder out from the trunk. The inner extremity is large and rests with its flat surface on the upper outer edge of the sternum, with the interposition of a disk of fibrocartilage. Its outer extremity is flattened; it articulates by means of a gliding joint with the acromion process of the scapula, and it is connected with the base of the coracoid process beneath by ligaments. It is double curved, the large curve having its convexity forward and embracing the inner two-thirds of the bone, and the small curve having its concavity forward, forming the outer third.

Fig. 230.   Right clavicle, upper surface.

Fig. 230. - Right clavicle, upper surface.

At the deepest part of the concavity of the anterior edge, about at the junction of its outer and middle thirds, is a small rough eminence called the deltoid tubercle because of the attachment to it of the deltoid muscle. At a corresponding point on the posterior and under surface of the bone is a prominent projection called the conoid tubercle; to this is attached the conoid ligament. Running forward and outward from this tubercle on the under surface is a rough line which serves as the point of attachment of the trapezoid ligament. Both these ligaments are of importance in relation to the fractures of this bone.

Fig. 231.   Right clavicle, under surface.

Fig. 231. - Right clavicle, under surface.

The middle third of the bone is its weakest part. Attached to the clavicle on its anterior surface are the deltoid muscle on its outer third and the pectoralis major on its inner half. On its posterior and upper surfaces are the trapezius at its outer third, and the clavicular head of the sternomastoid on its inner third. The subclavius muscle is attached to its under surface. It will thus be seen that there is a space equal to one-sixth of the length of the bone inferiorly and one-third of the bone superiorly which is free from muscular attachments, and it is here that it is most frequently fractured.

The Scapula

The scapula is spoken of as having a body, neck, spine, and acromion, glenoid, and coracoid processes; an upper, an anterior, and a posterior border; and an upper and a lower angle. It is not often spoken of as possessing a head, the glenoid process or that portion being sometimes so called in which the glenoid cavity or fossa for the articulation of the humerus is situated.

The constriction surrounding the head of the scapula is known as the anatomical neck, in contradistinction to the surgical neck, which name has been given to that portion indicated by a line drawn through the suprascapular notch and passing beneath the spine and to the inside of the attachment of the long head of the triceps muscle just below the lower edge of the glenoid cavity.

The angles and borders and spine are important landmarks in physical diagnosis and the coracoid and acromion processes in injuries.

In the upper border of the bone at its junction with the base of the coracoid process is a deep notch called the suprascapular notch (incisura scapulae), the supra scapular nerve passes through it. The artery passes over it. From the edge of the bone just behind the notch arises the omohyoid muscle, an important guide in operations on the neck.

Fig. 232.   Scapula, showing muscular attachments.

Fig. 232. - Scapula, showing muscular attachments.

The body of the scapula on its under surface is flat and rests on the thorax from the second to the seventh and nearly to the eighth rib. Its movements on the chest are free and follow those of the arm. It rises and falls, glides forward and backward, and also rotates on an anteroposterior axis.

When using any portion of the scapula as a landmark it is customary to have the arm hanging by the side; if it is otherwise the position of the bone will be changed, and the relations of its projections to the surrounding parts are altered. The scapula is sometimes fractured directly across its body below the spine. One should endeavor to fix in mind especially the relation of the acromion and coracoid processes to the head, with its glenoid cavity, and the rest of the bone.

The head is comparatively small and cup-shaped, with the glenoid fossa on its surface for the head of the humerus. It is joined to the body of the bone by a narrow constriction called the neck. Fractures through this neck are rare. Above and posterior to the glenoid fossa is the acromion process and spine of the scapula, and above and anterior is the coracoid process.

The spine of the scapula runs upward and forward across the upper and posterior surface of the bone. Its commencement at the posterior edge of the bone is called its root; this is over the fourth rib and opposite the third thoracic spine. The posterior edge of the scapula opposite the root of the spine projects backward, but this is not the superior angle, which is still higher up. The spine of the scapula ends anteriorly in the acromion process. This projects far beyond the glenoid cavity, overhangs the head of the humerus, and forms the point of the shoulder. It, as well as the rest of the spine, is subcutaneous and is a valuable landmark.

The acromion process is not so often fractured as one would expect. It articulates with the clavicle and the bones are not infrequently luxated at this point.

The coracoid process projects forward underneath the clavicle to the upper and inner side of the head of the humerus. It is about 2.5 cm. (1 in.) below the clavicle and just to the outer side of the junction of its middle and outer thirds. It lies just underneath the inner edge of the deltoid muscle, hence it is not always easily felt. It is almost never fractured, but is especially valuable as a landmark in injuries and operations on the shoulder.

The two great hollows above and below the spine are the supra- and infraspinous fosses for the supra- and infraspinatus muscles. The angles are at the two extremities of the posterior border. The superior angle receives the insertion of the levator scapulae muscle and is covered by the trapezius which inserts into the spine and thus obscures its outline. The inferior angle is crossed by the upper edge of the latissimus dorsi muscle, from which it sometimes receives a few fibres. This angle is rendered prominent when the forearm is flexed on the arm and placed across the back.