Limbs : Upper Limb 69 



On the other hand, it has been held that the bone is really the much modified anterior ventral 

 bar, corresponding with the precoracoid in the simple type of arch, and in support of this it can be 

 shown that there is a cartilaginous structure in the anlage of the bone immediately preceding its 

 ossification. Possibly the bone does not correspond with the " clavicles " that are found in some 

 lower animals. Ornithorhynchus is the only mammal that possesses coracoid bars that reach the 

 sternum, and if its " clavicle " is morphologically a separate bone it exists with a coracoid and a 

 precoracoid that is, if the epicoracoid of this animal represents the precoracoid of the simple type. 

 It is possible that the human clavicle is partly a covering dermal bone, and partly a primary ventral 

 bar which has been taken into the covering bone and is represented by the included cartilaginous 

 core in the ossifying structure. 



The thick compact bone of the shaft wall thins away toward the ends. The 

 association with longitudinal pressure, due to the pull of muscles going from the trunk 

 to the arm, is indicated by the well-marked longitudinal lamellae in the cancellous 

 tissue. 



The clavicle is well developed in mammals in which the fore limbs are used for 

 purposes beyond the mere to-and-fro movements of progression : thus functions that 

 call for lateral motions in the limb require a clavicle to support the scapula. 



This is seen in birds as well as mammals, for the flying birds have strong clavicles 

 firmly articulated with the sternum, whereas the cursorial birds have only rudimentary 

 bones. 



Among mammals it is well developed in those that use their front limbs for pre- 

 hension, as among the monkeys or the rodents, while animals like the horse and 

 carnivora have none or only rudimentary suggestions of the 

 bone embedded in the muscles of the shoulder. 



Development. At the end of the first month the bone is 



represented by a mesenchymal condensation lying obliquely l,Jk from below. 



below the future acromial region of the scapula, which is 

 placed in the neck. In the fifth to sixth week a couple of 

 centres of ossification appear in the rod of condensation, 

 which is now in a peculiar condition of " pre-cartilage. " 

 The two centres seem to indicate the development of the 

 shaft of the bone in two parts, and are preceded by the pre- 

 cartilaginous change occurring also in two separate areas. In 

 the seventh week the two areas and the two centres are 

 continuous, and from now on the spreading ossification is ^^ 

 preceded by cartilaginous changes in the mesenchyme. end of left clavicle. 



As a rule only one epiphysis develops, and this only 



forms a thin disc of articular surface (Fig. 55) at the inner end. The small size of 

 this epiphyseal plate can be understood when it is stated that its centre occurs very 

 late, about the twentieth year, when the bone is nearly fully grown, and fuses very 

 quickly. On the other hand, the centres for the shaft are the earliest centres to appear 

 in the body. 



SCAPULA, 



A " flat " bone, placed on the postero-external aspect of the upper part of the 

 thorax, connected with the sternum by the clavicle, and carrying the humerus. 



Triangular in shape, therefore with three borders and three angles, and a dorsal 

 and ventral surface (Fig. 56). Ventral surface concave, forming the subscapular 

 fossa, covered by and giving origin to Subscapularis : a raised rim along its inner 

 border for insertion of Serratus magnus. Dorsal surface presents the prominent spins, 

 projecting as a free process, the acromion, in a forward and upward direction externally : 



