This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The upper extremity is joined to the trunk by the shoulder-girdle, which is composed of the clavicle and scapula. The main movements are anteroposterior, as in swinging the arm, those of abduction and adduction, as in raising and lowering it sidewise, and rotation.
The scapula in the mole and many other animals may be a comparatively slender bone, but when, as in man, it is necessary to rotate the arm, then the scapula is large so as to form a strong support for the infraspinatus, supraspinatus, teres minor and major, and subscapularis muscles.
Fig. 227. - Shoulder-girdle of man.
The clavicle is developed mainly from membrane, partly probably from cartilage, and is the first bone in the body to ossify. It keeps the shoulder out away from the body and increases the range of motion of the upper extremity. It owes its existence to the function of abduction. Without a clavicle abduction is practically wanting and when in man the clavicle is broken, he is temporarily reduced to the condition of those animals which have no clavicles; he is able to move the arm backward and forward but not to elevate it properly, and this is an important diagnostic symptom of that injury.
The clavicle is lacking in the ungulates or hoofed animals. These have an anteroposterior movement, but little abduction. A horse or cow moves its fore-legs back and forward, but not out away from the body. Hence its helplessness when these movements are essential. It is also lacking in seals and whales. In the carnivora, as the lion and the tiger, which possess rudimentary clavicles, sufficient adducting power is present to enable them to hold their food while tearing it apart. In man, apes, bats, rodents, and insectivora the clavicle exists as a well-formed bone;
Fig. 228. - Shoulder-girdle of man, showing how the clavicle acts as a prop to keep the shoulder out away from the chest.
hence they can raise the arm well out from the body and even higher than the shoulder. In the rodents, as the squirrel, they are enabled to hold a nut firmly in the paws while eating it. When, as in some of the lower orders, the function of abduction is all important, we find not only the clavicles present and, as in the common fowl, joined, forming the "wish-bone," but in addition, in birds, there is a precoracoid bone formed by the coracoid process, which is enlarged and continued forward to articulate with the sternum; thus in flying animals there are practically two clavicles on each side.
The point of the shoulder projects well out from the side of the thorax. Hence it is frequently injured. As the force is resisted by the bones, these receive the principal injuries and they are often broken. Fractures of the clavicle dispute with those of the radius the distinction of being the most numerous. Contusions produce more or less complete paralysis of the muscles, not infrequently through lesions of the nerves. The laxity of the joint favors the dislocations to which it is so frequently subject. It likewise becomes the seat of tuberculous disease requiring resection. Crushes of the arm sometimes require its removal at the shoulder-joint, and occasionally as the result of injury or disease operations may be required on the axillary lymph-nodes, nerves, or blood-vessels.
Fig. 229. - Shoulder-girdle in birds. Skeleton of an eagle, from the Wistar Institute: the clavicle, precoracoid, and scapula form the shoulder-girdle; the two clavicles have fused in the median line, forming what is commonly-called the " wish-bone".
In order to determine the character and extent of injuries to the shoulder, its surface anatomy must be thoroughly known. In order to treat them, a knowledge of the deeper structures and their relation to one another is essential.
The landmarks of the shoulder are formed by the bones and muscles; hence a brief review of their important characters will serve as a basis for the surface anatomy which follows.