50 



THE BIOLOGY OF BIRDS 



the power of flight. The strong development of the keel 

 and of the breastbone as a whole may be associated with 

 a characteristically avian lightening of the bone, and with 



Fig. 6. — Breastbone and pectoral girdle of a penguin. From a 

 specimen. ST., sternum ; K., keel ; xp., xiphoid process of sternum ; 

 M., manubrium or anterior process of sternum ; sc, scapula or shoulder 

 blade ; CO., coracoid ; F., foramen in coracoid ; H., head of humerus work- 

 ing in the glenoid cavity where scapula and coracoid meet ; f.tr., fora- 

 men triosseum bounded by scapula, coracoid, and clavicle — a pulley hole 

 for the tendon of a pectoral muscle ; CL., the clavicle ; l.CL., interclavicle. 



its splitting up into (xiphoid) processes. Another adaptive 

 feature is that the backward elongation of the breastbone 

 forms a useful supporting floor for the abdominal viscera — 



