Duval's artistic anatomy

The shoulder is formed of three bones, of which one is situated in the front, the collar-bone, or clavicle; a second is behind, the shoulder-blade, or scapula; and a third is placed externally, the upper part of the arm-bone, or humerus.

The clavicle (clavicula, diminutive of clavis, a key) is a long bone, placed horizontally at the junction of neck and chest. It connects the breast-bone with the shoulder-blade; and the two clavicles are separated in the middle line by the upper end of the manubrium, or pre-sternum (episternal or suprasternal notch). Its form is that of an italic ∫—that is to say, it describes in a horizontal plane two curves, the internal portion being convex in front, and the outer part convex behind (Fig. 16). It consists of a shaft and two extremities: the shaft, flattened from above down, presents a smooth superior surface, which in the model shows very clearly beneath the skin, and a rough inferior surface for the attachment of a muscle (subclavius), and for the attachment of ligaments connecting it with the first rib internally, and with the coracoid process of the scapula externally (see below); it possesses a posterior and an anterior border, thick and curved: an internal, sternal extremity, thick and triangular, which articulates with the corresponding lateral facet on the manubrium of the sternum: and an outer, acromial extremity, flattened and presenting an oval articular surface for articulation with the acromion process of the scapula. The clavicle thus serves to connect the scapula to the trunk, and by the articulations of its two extremities it increases, as we shall see, the mobility of the shoulder-joint. We find that the clavicle, which is almost horizontal when the arms are hanging in an attitude of repose beside the trunk, becomes oblique in direction when the arms are moved upward and forward, or upward and backward. The length of the clavicle, as we have already said (p. 46), should be equal to that of the sternum without the xiphoid appendix.

The shoulder-blade, or scapula, is a flat, triangular bone, and consists of a bony plate very thin at certain points, and thickened only along its borders. It is placed on the lateral and posterior aspect of the thorax, and corresponds at its upper border to the second rib: its lower end reaches to the seventh or eighth rib. It is attached by a joint called the acromio-clavicular articulation, which can be felt beneath the skin, to the external extremity of the clavicle. As this joint is the only ligamentous connection of the scapula with the trunk, the bone is capable of great freedom of movement.

The shoulder-blade is described as possessing two surfaces, three borders, three processes, and three angles. The posterior surface, free and visible throughout its entire extent upon the mounted skeleton, is divided into two unequal portions—the upper one, the smaller (supra-spinous fossa) and the lower one, the larger (infra-spinous fossa)—by a bony crest, called the spine of the scapula (10, 11, Fig. 17), which traverses the bone obliquely from within upwards and outwards. It becomes more and more prominent externally, and is afterwards prolonged into a free process, broad and flat, which forms the most elevated and external part of the skeleton of the shoulder (12, Fig. 17), and is known by the name of the acromion process (ἃχρος, summit; ὦμος, the shoulder). It is on the internal or anterior border of this acromion process that the small oval facet is found, by which the scapula articulates with the outer extremity of the clavicle by the acromio-clavicular articulation. The fossa above the spine of the scapula is the supra-spinous fossa (1, Fig. 17), and the larger space below the spine is called the infra-spinous fossa (2, Fig. 17). The anterior surface of the scapula has received the name of the subscapular fossa. It is directed forwards and inwards, and is separated from the ribs and armpit by large muscles. It is but little visible in the articulated skeleton.

Of the three borders of the scapula, one is superior and horizontal; it is the shortest of the three. The second is internal and vertical, parallel to the series of spinous processes of the vertebræ; to this is given the name of the vertebral border. The third, or outer border, is oblique, and is directed downwards and backwards; it corresponds to the region of the armpit, and has received the name of the axillary border. It is particularly to be noticed that the vertebral border is thin; the axillary border, on the contrary, is thick, and presents towards its superior limit a rough space, just below the glenoid fossa, for the attachment of the long head of the triceps muscle (8, Fig. 17). The superior border is remarkable for the presence at its outer extremity of a stout process which projects at first upwards, and is afterwards bent outwards upon itself like a bent finger. It has been likened to the beak of a crow, and has received the name of the coracoid process (κὸραξ, a crow; εἷδος, form). The coracoid process (14, Fig. 17) is placed in front and on the inner side of the acromion, and the two together form an arch over the shoulder-joint, called the coraco-acromial arch, which is completed by a fibrous band proceeding from one process to the other, and called the coraco-acromial ligament. The position of the coracoid process should be particularly noted in relation to the clavicle. It underlies the outer part of the clavicle, from which two important ligaments (conoid and trapezoid ligaments) descend to be attached to its upper surface. These ligaments serve the purpose of slinging the scapula up to the under surface of the clavicle. Of the three angles of the scapula, one only—the superior external angle, which is situated beneath the coraco-acromial arch—deserves a particular description; it is very thick, and becomes enlarged to form an articular surface (6, Fig. 057), which is directed outwards and forwards, and articulates with the head of the humerus. This surface is pear-shaped and is slightly hollowed out, and bears the name of the glenoid cavity. It is much less extensive than the articular head of the humerus, along with which it forms the shoulder-joint.

In regard to the relative proportions of the scapula, it may be noted that the length of the vertebral border is equal to the length of the clavicle, and equal also to the distance which separates the scapulæ behind, when the vertebral borders are vertical, which is the case when the arms are hanging beside the body in a position of relaxation.

The movements of the shoulder-blade, and their effects on external form, will be studied in relation to the articulation of the scapula with the humerus, the scapulo-humeral articulation, or shoulder-joint. We must now pass on to describe the upper portion of the bone of the arm.

The arm bone, or humerus, is one of the long bones, and is composed of a shaft, prismatic or nearly cylindrical in form, and of two enlarged extremities, one inferior, which takes part in the articulation of the elbow; the other superior, which takes part in the articulation of the shoulder. We will only concern ourselves for the present with the superior extremity.

The superior extremity of the humerus is large and spherical. It is continuous with the body of the bone by a cylindrical neck, called the surgical neck of the humerus. This extremity is traversed by a circular groove, oblique from above downwards, and from without inwards, called the anatomical neck (3, Fig. 18). This is well marked, and divides the sphere into two parts; the one, situated above, and internal to the anatomical neck, is very regularly rounded, smooth, covered over by a layer of cartilage, and is called the head of the humerus (2, Fig. 18); it is normally in contact with the glenoid cavity of the shoulder-blade, in which it glides in the movements of the arm; the other part of the head, situated below, and external to the anatomical neck, is rough, and divided into two tuberosities by a vertical groove, which is prolonged as far as the upper part of the body of the bone, and which, from its serving to accommodate the long tendon of the biceps, has received the name of the bicipital groove (6, Fig. 18). The tuberosity situated on the outer side of the bicipital groove is large, and is called the great tuberosity (4, Fig. 18); it presents three small surfaces which receive the insertions of the deep muscles of the shoulder—supraspinatus, infraspinatus, and teres minor muscles. The tuberosity situated on the front of the bone, and internal to the bicipital groove, is smaller. It is called the small tuberosity of the humerus (5, Fig. 18), and gives attachment to one muscle only, the subscapularis.

The Shoulder-joint.—The articulation of the shoulder, or scapulo-humeral articulation, may serve as a type of articulations in general. It is necessary in each articulation to consider how the shapes of the bony surfaces in contact correspond to each other. From this we should be able to deduce the nature of the movements permitted by the articulation. It is also necessary to consider the disposition of the ligaments; that is to say, of the fibrous bands which proceed from one bone to the other, and from these we can deduce the limits of the movements of which the joint may be capable.

The scapulo-humeral articulation is a ball-and-socket joint, formed by the glenoid cavity of the shoulder-blade and the head of the humerus.

Such an arrangement of surfaces in contact with one another permits the head to glide in all directions within the cavity, and consequently the upper limb can be moved in all directions: forwards; backwards; inwards (adduction); outwards (abduction); upwards and downwards. The head of the humerus can also be rotated on the glenoid cavity, either inwards or outwards.

The ligamentous covering of the joint is formed by a fibrous capsule, which is attached on one side to the margin of the glenoid cavity, and on the other side to the anatomical neck of the humerus (5, Fig. 19).

This ligamentous capsule is sufficiently loose to allow the head of the humerus great freedom of movement within the glenoid cavity, without any part of the capsule being put on the stretch, and thus stopping its movement. Thus the movement of the arm forwards is very extensive, as are those backwards and directly inwards, this last being checked only by the meeting of the arm with the lateral surface of the trunk. But the movement of abduction, or of elevation outwards, becomes difficult when the arm approaches the horizontal; here an arrangement comes into play which is of great importance, the study of which shows us that the upper limb, besides the mobility which belongs to the shoulder-joint proper (scapulo-humeral articulation), derives an increase of mobility from the articulations of the shoulder-blade with the clavicle, and of the clavicle with the sternum.

When we apply a humerus to the glenoid fossa of the scapula in such a manner as to draw the arm outwards, we observe that at the moment when the humerus attains the horizontal position, the tuberosities touch the coraco-acromial arch which surmounts the glenoid cavity, and which is completed by the coraco-acromial ligament (3, Fig. 19); in the complete subject, in the living man, when the arm is elevated in carrying it outwards, the same effect of meeting and, so to speak, of contact between the tuberosities of the humerus and the acromial arch is produced, with this result—that the arm, once horizontal, is with difficulty raised higher by the simple play of the scapulo-humeral articulation—that is to say, by the gliding of the head of the humerus in the glenoid cavity. But now a new source of mobility is employed, a new articulation is brought into play—that of the acromio-clavicular joint. The entire scapula moves round the extremity of the clavicle; its inferior angle is carried forward; its external angle—that is to say, the glenoid cavity with the coraco-acromial arch—is carried upwards, and the movement of elevation of the arm is then continued by the play of the shoulder-blade.

As the result is an important change of shape in the shoulder, we ought to particularly examine the shoulder in the region of the back, upon a model in which the arm is raised above the horizontal position. The point of the shoulder is raised, and as this elevation is accompanied by a movement of the shoulder-blade, the vertebral border of this bone does not remain parallel to the spinal column, but while it approaches closer to it at its upper end, it is drawn further away at its lower end, and is obliquely directed from above downwards and outwards (Fig. 20). The inferior angle becomes prominent in the lower part of the armpit, as we may observe in a subject with the arms folded, and shows in a striking manner on a dead crucified body. If the elevation of the arm approaches the vertical, the vertebral border of the shoulder-blade has a tendency to become horizontal, and we observe this change of movement in the model when the bone is seen along the posterior surface of the shoulder and the back, and we should scarcely recognise at first the appearances presented as those we are accustomed to study when it occupies its ordinary situation in the skeleton.

But when the arm is elevated vertically, so that it comes in contact with the side of the head, movement takes place not only at the scapulo-humeral and at the acromio-clavicular joints, but also at the articulation of the internal extremity of the clavicle with the sternum. In that case, in fact, the whole of the shoulder is raised upwards (by the superior fibres of the trapezius muscle), and the clavicle represents the arm of the lever by which this movement is accomplished, while the sterno-clavicular articulation is the hinge. Thus we see the clavicle is changed in its direction, from the horizontal, to become oblique upwards and outwards—that is, its external extremity is raised and carried a little backwards.

The clavicle plays a very important part in the movements of the upper limb. This explains why it is found specially developed in those animals whose fore-limbs possess the most specialised movements—flight, digging, burrowing, striking, or hugging—such as man, the monkey, the bat, the tiger, the mole, etc.; while in those quadrupedal animals which, like the horse, use their fore-limbs for progression only, and in which movements occur only in a plane parallel to that of the body, there is not a trace of clavicle. The part which the clavicle takes in the movements of the arm explains also why the bone presents a variable size in different individuals. It is stronger in the male than in the female; stronger in the working man than in the student; stronger, finally, on the right side than the left, from the habit of using the right arm for the most part in those actions which demand strength and skill. In the left-handed, it is the left clavicle that is stronger than the right. In a word, this bone is, like all the other parts of the skeleton, stronger in proportion as it takes part in active and frequently repeated movements. Thus the breadth of the shoulders is one of the characteristics of athletes; and it is to the strength of the bony structure formed by the clavicle and shoulder-blade, sustained by the first ribs, that the superior portion of the thorax owes its characteristic aspect.

Owing to the presence of the shoulder girdle (scapula and clavicle) the thoracic cage does not present much of the form of a cone at its upper end, or summit; indeed, this region becomes broader in a lateral direction as the clavicle is more developed. It will be enough to contrast the region of the thorax in man with that of such animals as the dog or the horse, which, owing to the absence of a clavicle, have a thorax transversely flattened in the region of the shoulders, and consequently the shoulder-blades more closely applied upon the sides of the thoracic cage.

This is the proper place to discuss the dimensions and proportions of the shoulder; but, as this study consists entirely of a comparison of the transverse diameters of the shoulders with the transverse diameters of the hips, it is better to defer it till after the description of the pelvis.