EXTREMITY. 



155 



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 

 subclavius 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 coraco-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 pectoral is 

 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 formed between the 

 axilla and the neck. 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 enter 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 Carnivora, 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. When 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. He 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 Mammalia that dig and burrow in 

 the ground, or whose anterior extremities are 

 so modified as to aid them in flight, or who 

 are skilful in seizing upon and holding objects 



* Bridgewater Treatise, p. 48. 



