160 



Dynamic Theory. 



Whenever the fore limbs are pulled forward over the head or across 

 the chest, or at any angle between the two, a compression strain is 

 thrown upon the parts occupied by these bones ; but when the limbs are 

 pushed in the contrary direction the compression strain is thrown upon 

 the scapula. It is obvious, therefore, that the movements of the front 

 limbs in swimming, flying and climbing, or swinging by the hands, will 

 tend to develop either the clavicle or coracoid, or both, while walking 

 on the fore limbs will develop only the scapula. The cartilaginous 

 fishes, Sharks, Rays, Chimera, &c., furnish a sample of one of the 

 earliest forms of the pectoral arch. The bones are not ossified and the 

 three bones on each side are represented by a single piece of cartilage. 

 These two lateral cartilages are joined together in the middle line in 

 front. They therefore occupy the territory and serve the same pur- 

 poses that are accomplished better in the more advanced development of 

 the parts. In the Teleostean, or bony fishes, the divisions of the pec- 

 toral arch are, in general, fairly established, and the scapula coracoid 

 and clavicle are distinct from each other. There is, however, more or 

 less variation in forms, often some additions and membranous append- 

 ages, and often the clavicle is not ossified but remains a membrane bone. 

 In the mud-fishes ( Lepidosiren ) the pectoral arch is chiefly cartilagi- 

 nous, but the different parts practically represent the same pieces as they 

 occur in the other fishes. ( Their fins are long and thread-like. ) 



The Amphibians ( frogs ) also have all the parts, but the coracoid is 

 much more largely developed than the clavicle. It is, in fact, a broad, 



FIG. 83. Sternum and Pectoral arc.h 



of Frog, as seen from above, the left 

 supra scapula removed. 



sc. Scapula. 



ssc. Supra scapula. 



psc. Prescapular process. 



cr. Coracoid. 



ecr. Epicoracoid. 



erf. Coracoid fontanelle. 



pcrc. Precoracoid with clavicle, 



ost. Omosternum. 



xst. Xiphisternum a sword shaped 

 cartilaginous appendage to sternum. 



gl. 'Glenoid cavity, place of attach- 

 ment of arms. 



triangular plate with its base on the median line of the chest and its 

 apex articulating with the scapula. It contains a fontanelle or open 

 space at a point obviously free from strain, and its anterior edge is 

 bounded by the clavicle. 



The Lizzards generally possess well developed clavicles and coracoids, 

 but the Crocodiles have only the coracoids, and are devoid of clavicles. 



