258 



HUMAN ANATOMY. 



Fig. 277. 



external to this the rough concave edge gives origin to the deltoid. The beginning 

 of this is often marked by a minute tubercle, which, when exceptionally large, is 

 the deltoid tubercle. The posterior surface is smooth, and narrows gradually till 

 it reaches the outer end, the beginning of which is marked by a tubercle on the 

 under surface. 



The borders are very ill marked. The sharpest is that separating the anterior 

 from the inferior surface. That between the anterior and superior ones is fairly well 

 marked near the inner end ; but it soon grows indistinct, so that often at the middle 

 of the bone the front surface seems to twist into the upper, and the anterior inferior 

 border becomes the front border of the outer end. Of the posterior borders, the 

 upper, though rounded, is distinct along the middle of the bone ; the lower is very 

 vague, but usually is well defined in the outer part ; when it is not, the posterior 

 surface seems to twist into the lower. 



The inner or sternal extremity ^ is club-shaped, drawn out downward and 

 somewhat backward. Its inner surface, coated with articular cartilage, is of very 

 variable shape. It is approximately oval, with the long axis slanting downward and 

 backward, and is rough and generally concave, but not always so. The front edge 

 of the inner surface forms an acute angle with the anterior border of the bone, and 

 the hind one an obtuse angle. 



The outer or acromial extremity ' is flattened above and below and curved 



forward. At the very front of this end is 

 an articular surface joining the scapula. It 

 is oval, with the long a.xis horizontal, and 

 usually faces downward as well as outward. 

 There is generally behind this a rough space 

 for ligament at the end, which gradually 

 slants into the hind border. The co7ioid 

 tubercle^ is at the posterior border of the 

 lower surface of the outer extremity just at 

 its junction with the shaft. The trapezoid 

 ridge extends from it forward and outward 

 across the bone. Its anterior portion is 

 often broad. This tubercle and ridge are 

 for the insertion of ligaments of correspond- 

 ing names, passing upward from the base of the coracoid. Between the ridge and 

 the end of the bone there is a smooth space, sometimes almost a groove, which lies 

 above the supraspinatus muscle as it crosses the shoulder-joint. The clavicle varies 

 greatly in length, thickness, amount of curve, and in the outline of the ends. Some- 

 times the outer end is but little broader than the shaft. A very rare form is one in 

 which the inner part of the shaft is flat and but slightly thicker than the outer. 



Differences due to Sex. — The male bone is longer, stronger, more curved, 

 and with larger articular facets. Apart from sex, a strong bone is generally more 

 curved. 



Development. — The centre for this bone, evident in the sixth week, precedes 

 all others. It is remarkable as developing in indifferent tissue before any hint of the 

 bone is to be seen. A little later a cartilaginous outline appears, which is shortly 

 involved in the ossifying process. At about seventeen a centre is formed in the 

 epiphysis at the sternal end, and joins the shaft a year or so later. 



Surface Anatomy. — In life the anterior surface and edge, as well as the 

 superior surface, are easily felt through the skin. The joint with the acromion is dis- 

 tinct, the clavicle being higher than the scapula. The position of the bone is nearly 

 horizontal, but in strong men the outer end is often the higher. The bone is highly 

 elastic, owing to its curves. 



Ossifiealion of clavicle. A, at birth ; a, chief cen- 

 tre ; 6, f , cartilaginous ends. B, at about eighteen 

 years ; rf, sternal epiphysis. 



PRACTICAL CONSIDERATIONS. 

 The chief function of the clavicle is to steady the shoulder and keep the upper 

 arm at such a distance from the trunk that the muscles running from the latter to 

 the humerus may give it lateral motion. Therefore, in animals, in which no such 



^ Extremitas stemalis. - Extremitas acromialis. •''Tuberositas coracoidea. 



