190 



ANIMAL LOCOMOTION. 



The power of the bird is thus concentrated for the purpose of 

 moving the wings and conferring steadiness upon the volant 

 mass. In birds of strong flight the keel is very large, in 

 order to afford ample attachments for the muscles delegated to 

 move the wings. In birds which cannot fly, as the members 

 of the ostrich family, the breast-bone or sternum has no keel.^ 



The remarks made regarding the muscles of birds, apply 

 with very slight modifications to the muscles of bats. The 

 muscles of bats and birds, particularly those of the wings, 

 are geared to, and act in concert with, elastic ligaments or 

 membranes, to be described presently. 



Lax condition of the Shoulder-Joint in Bats, Birds, etc, — The 

 great laxity of the shoulder-joint in bats and birds, readily 

 admits of their bodies falling downwards and forwards during 

 the up stroke. This joint, as has been already stated, admits 

 of movement in every direction, so that the body of the bat 

 or bird is like a compass set upon gimbals, i.e. it swings and 

 oscillates, and is equally balanced, whatever the position of 

 the wings. The movements of the shoulder-joint in the bird, 

 bat, and insect are restrained within certain limits by a 

 system of check ligaments and prominences ; but in each 

 case the range of motion is very great, the wings being per- 

 mitted to swing forwards, backwards, upwards, downwards, 

 or at any degree of obliquity. They are also permitted to 

 rotate along their anterior margin, or to twist in the direction 

 of their length to the extent of nearly a quarter of a turn. 

 This great freedom of movement at the shoulder-joint enables 

 the insect, bat, and bird to rotate and balance upon two 

 centres — the one running in the direction of the length of the 

 body, the other at right angles or across the body, i.e. in 

 the direction of the length of the wings. 



In the bird the head of the humerus is convex and some- 

 what oval (not round), the long axis of the oval being directed 

 from above downwards, i.e. from the dorsal towards the ven- 

 tral aspect of the bird. The humerus can, therefore, glide up 

 and down in the facettes occurring on the articular ends of the 



1 One of the best descriptions of the bones and muscles of the bird is that 

 given by Mr. Macgillivray in his very adnairable, voluminous, and entertain- 

 ing work, entitled History of British Birds. Lond. 1837. 



