CHAPTER V 



THE CLAVICLE 



One other skeletal element of the fore-limb needs brief 

 mention, and this is the collar-bone, or clavicle, which is 

 so well developed in Man. Although the homologues of 

 the clavicle are perhaps more debated than those of any 

 other bone in the body, it is not proposed to enter into 

 any discussion regarding the respective merits of those 

 theories which find the caudal homologue of the clavicle 

 now in one, and now in another, element of the pelvic 

 girdle. Here the clavicle will be regarded, as the work 

 of Fawcett appears clearly to indicate, as an element 

 peculiar to the fore-limb girdle. We will assume that 

 the bone of the clavicle is an intramuscular ossification, 

 making its appearance in the embryo first (at any rate 

 in the Mammals), by two ossific centres, the one (lateral) 

 in the deltoid-trapezius muscle sheet, and the other 

 (median) in the pectoralis-sterno-cleido-mastoid sheet. 

 These two ossific intramuscular intersections become 

 continuous, and, in its complete development, the bony 

 bar thus formed articulates at its median end with the 

 sternum and at its lateral end with the shoulder girdle 

 proper. We will further assume that this bony bar is 

 a purely functional development, that it is laid down as 

 a firm strut which keeps the shoulder girdle poised at 

 the sides of the body, and which makes an acting point for 

 the separated muscles that are derived from the sheets 

 in which it is laid down (see Fig. 9). As such a strut, 

 the clavicle is a very ancient possession of the Vertebrates. 

 It occurs in the Dipnoi and certain other primitive fish. 



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