THE MUSCULAR SYSTEM 37 



forms an outstanding feature in the mechanism 

 of the wing. An idea of the magnitude of 

 this mass may be gathered from the statement 

 that the muscles composing it weigh about as 

 much as do all the rest of the muscles of the 

 body together, and that they contribute about 

 one-twelfth of the weight of the entire body. 

 The most important members of the pectoral 

 group are two in number — the pectoral proper 

 and the supracoracoid. 



The pectoral muscle has an extensive origin 

 from the clavicle, the ligamentous membrane 

 joining the clavicle to the sternum, a narrow 

 strip of the sternal crest close to its ventral 

 edge, the posterior-lateral process (including 

 its oblique process) of the sternum and the 

 membrane between this process and the meta- 

 sternum. A few fibres also arise from the 

 ribs close to the oblique process. Convergence 

 of the fibres towards the shoulder- joint allows 

 of insertion into the projecting greater (lateral) 

 tubercle near the head of the humerus. The 

 action of the muscle is to depress the wing. 



The supracoracoid muscle is entirely covered 

 by the preceding, compared with which it is 

 much smaller. Its origin is from the whole 

 of the sternum (including the rostrum) not 



