The internal extremity, also called sternal, is formed by a gradual expansion of the shaft of the bone, which, however, still preserves the general cylindrical form, but is flattened a little on its superior surface in size this ex ceeds all other parts of the bone. The inner surface of this extremity of the clavicle is destined for articulation with the sternum, and accordingly we find on it a considerable arti cular facet, which is convex from above down wards, and concave from before backwards. The outline of this surface is triangular, and each angle is easily distinguishable by the degree of its prominence : thus, one angle is situated anteriorly and inferiorly, it is the least prominent ; a second is posterior and inferior, it is the most prominent ; and the third is su perior, and may easily be felt under the inte guments in the different motions of the bone. The external or acromial end of the clavicle is at once distinguished by its flattened appear ance; it is flattened on its superior and in ferior surfaces. At its extremity we find an elliptical articular surface adapted to a similar one upon the acromion process ; this surface is nearly plane, its long axis is directed horizon tally from before backwards.
The body or shaft of the bone presents se veral points deserving of notice. The superior surface is smooth and rounded, expanding to wards the sternal end, where it affords attach ment to the clavicular portion of the sterno mastoid muscle. It expands likewise towards the acromial end, but loses the cylindrical form and becomes flattened : the central part is the most contracted and the most cylindrical; here the bone is almost subcutaneous, being co vered only by the common integument, some fibres of the platysma, and crossed by the supra-clavicular filaments from the cervical plexus of nerves.
On the inferior surface of the clavicle we notice towards its sternal end a rough surface for the insertion of the costo-clavicular or rhom boid ligament : external to this and extending outwards is a superficial excavation along the inferior surface of the bone, which lodges the subelavins muscle. This groove terminates at the commencement of the external fourth of the bone, where we notice a rough and promi nent surface for the insertion of the comeo-cla vicular or conoid and trapezoid ligaments; in the articulated skeleton this surface corresponds to the root of the coracoid process, immediately over which it lies. On the inferior surface, near its middle, is the orifice of the canal for the transmission of the nutritious artery, the direction of which is outwards.
The anterior edge is thicker and more rounded towards the inner than towards the outer end, where it partakes of the general flattened ap pearance of the bone at that part; in the former situation it affords attachment to the pectoralis major muscle—in the latter to the deltoid. The two internal thirds of this edge are convex, its external third is concave.
The posterior edge is smooth and thin upon its two internal thirds, thicker and rougher at its external third, where the trapezius muscle is inserted into it; in the former situation this edge is convex, in the latter it is concave. The relations of the clavicle in this situation are in teresting : it forms the anterior boundary of a space somewhat triangular in form, through which a communication is Ibrmed between the axilla and the tied'. The posterior boundary of this opening Is formed by the superior border of the scapula, and the internal by the inferior vertebra of the cervical region of the spine, while the first rib constitutes a sort of floor, over which pass the various vessels, nerves, and other parts which cuter the cavity of the axilla. The anterior third of the first rib passes
beneath the sternal end of the clavicle, but its two posterior thirds lie on a plane superior to it. Consequently we find that the cone of the pleura passes up behind this end of the clavicle so as to be on a level with it, hence the so noriety elicited by percussion of the clavicle, and hence likewise the possibility in many instances, where embonpoint does not interfere, of hearing the respiratory murmur in the supra clavicular region.
The great importance of the clavicle in the motions of the upper extremity is rendered abundantly evident by observing how com pletely synchronous are its movements with even the slightest change of position in the arm. But this is illustrated in a more striking man ner by reference to the comparative anatomy of this bone. Those animals only possess a well developed clavicle whose habits of life require extensive and varied movements of the shoul der. Where the anterior extremity is employed merely as an instrument of progressive motion on a plane surface, we have no clavicle ; hence this bone is absent from the skeletons of Pa chydermata, Ruminantia, Solipeda, and the mo tions of the shoulders are only such as are required for the flexion and extension of the limb. In the Carnivore, where there is a slight increase in the range of motion of the anterior extremities, a rudimentary clavicle exists, and in this class we observe that the size of the bone in the different orders bears a direct relation to the extent of motion enjoyed by the limb. Thus it is smallest in the Dogs and largest in the Cats; in these animals it has no attachment to either the sternum or the scapula, but is enclosed in the flesh, and does not occupy much more than half the space between the two bones last named. " But, however imperfect," says Sir C. Bell, "it marks a correspondence in the bones of the shoulder to those of the arm and paw, and the extent of the motion enjoyed. NVben the bear stands up, we perceive, by his ungainly attitude and the motion of his paws, that there must be a wide difference in the bones of his upper extremity from those of the ruminant or solipede. Ile can take the keeper's hat from his head and hold it; he can hug an animal to death. The ant-bear especially, as he is defi cient in teeth, possesses extraordinary powers of hugging with his great paws; and, although harmless in disposition, he can squeeze his enemy the jaguar to death. These actions and the, power of climbing result from the structure of the shoulder, or from possessing a collar-bone however imperfect." In those Maminalia that dig and burrow in the ground, or whose anterior extremities arc so modified as to aid them in flight, or who are skilful in seizing upon and holding objects with their paws, the clavicle is fully developed, and extends the whole way from the scapula to the sternum. Thus in the Rodentia this bone is very perfect, as, for example, the Squirrel, the Beaver, the Rabbit, the Rat, &c. The Bat affords an example of a very strong and long clavicle, as also the Mole and the Hedgehog among the Insectivora.