ADAPTATIONS OF THE SKELETON 53 



rudimentary or absent in Running Birds ; it is rudimentary 

 in some birds that do not fly much, e.g. certain parrots. 

 The very strong coracoids also form a part of the fulcrum 

 for the stroke, and along with the merrythought they must 

 also protect the heart against compression. Of great 

 importance is the way in which breastbone, pectoral girdle, 

 ribs, and thoracic vertebras are all bound together into a 

 coherent elastic basket, on which the wings work ; the 

 compression of this basket, as the backbone is lowered and 

 the breastbone is raised, affects the lungs, which are firmly 

 attached to the ribs, and thus flying helps breathing. Hilz- 

 heimer calls attention to the mobility of the joint between 

 the coracoid and the breastbone in humming birds, and 

 connects it with the rapidity of the wing-strokes. It 

 allows of an unusually marked and rapid expansion and 

 compression of the body-cavity, thus facilitating respiration. 



(4) A very clear instance of adaptation is seen in the 

 relation of the elevator muscle of the wing to the breastbone 

 on the one hand and the humerus on the other. This 

 muscle that raises the wing — the pectoralis minor — is 

 inserted on the ventral surface of the breastbone. How 

 can it, lying ventrally, by contracting, raise the wing, 

 which is high above it ? How can a sailor on the deck of a 

 ship pull a sail upwards by hauling a rope downwards ? 

 Obviously, the rope must work over a pulley, and so is it in 

 the case of the tendon of the muscle. It passes upwards 

 alongside of the coracoid to the foramen triosseum (to the 

 internal side of the shoulder-joint), where scapula, coracoid, 

 and clavicle touch one another and enclose a hole. The 

 tendon passes through this pulley hole and is attached to 

 the dorsal surface of the humerus. Thus when the muscle 

 down below contracts, its tendon pulls the wing up. 



(5) In adaptation to flight remarkable changes have 

 come about in the skeleton of the fore-limb. There is 

 greatly reduced mobility in the different parts and the 

 wing works as a unified whole. When at rest the wing 

 forms a compressed letter Z, the elbow pointing backwards, 

 the wrist joint forwards, the tips of the three digits (probably 



