178 



ANIMAL LOCOMOTION. 



ishing and increasing the angle which its under surface makes 

 with the horizon, but to elevate and depress the wing, and 

 move it in a forward and backward direction. The rotatory 

 or twisting movement of the wing is an essential feature in 

 flight, as it enables the bat (and this holds true also of the 

 insect and bird) to balance itself with the utmost exactitude, 

 and to change its position and centre of gravity with mar- 

 vellous dexterity. The movements of the shoulder-joint are 

 restrained within certain limits by a system of check-ligaments, 

 and by the coracoid and acromian processes of the scapula. 

 The wing is recovered or flexed by the action of elastic liga- 

 ments which extend between the shoulder, elbow, and wrist. 

 Certain elastic and fibrous structures situated 'between the 

 fingers and in the substance of the wing generally take part 

 in flexion. The bat flies with great ease and for lengthened 

 periods. Its flight is remarkable for its softness, in which 

 respect it surpasses the owl and the other nocturnal birds. 

 The action of the wing of the bat, and the movements of 

 its component bones, are essentially the same as in the bird. 



The Wings of Birds. 



The Bones of the Wing of the Bird — their Articular Sur- 

 faces^ Movements, etc. — The humerus, or arm-bone of the 

 wing, is supported by three of the trunk-bones, viz. the 

 scapula or shoulder-blade, the clavicle or collar-bone, also 

 called the furculum} and the coracoid bone, — these three 

 converging to form a "point d'appui, or centre of support for 

 the head of the humerus, which is received in facettes or 

 depressions situated on the scapula and coracoid. In order 

 that the wing may have an almost unlimited range of motion, 

 and be wielded after the manner of a flail, it is articulated to 

 the trunk by a somewhat lax universal joint, which permits 



1 The furcula are usually united to tlie anterior part of the sternum by 

 ligament ; but in birds of powerful flight, where the wings are habitually 

 extended for gliding and sailing, as in the frigate-bird, the union is osseous in 

 its nature. *'In the frigate-bird the furcula are likewise anchylosed with 

 the coracoid bones." — Comp. Anat. and Phys. of Vertebrates, by Prof. Owen, 

 vol. ii. p. 66. 



