192 



ANIMAL LOCOMOTION. 



the stretch in extension, and which recover their original forrn 

 and position in flexion (fig. 98, c, p. 181). This simple ex- 

 periment shows that the various inclined surfaces requisite 

 for flight are produced by the mere acts of extension and 

 flexion in the dead bird. It is not, however, to be inferred 

 from this circumstance that flight can be produced without 

 voluntary movements any more than ordinary walking. The 

 muscles, bones, ligaments, feathers, etc., are so adjusted with 

 reference to each other that if the wing is moved at all, 

 it must move in the proper direction— an arrangement which 

 enables the bird to fly without thinking, just as we can 

 walk without thinking. There cannot, however, be a doubt 

 that the bird has the power of controlling its wings both 

 during the down and up strokes ; for how otherwise could 

 it steer and direct its course with such precision in obtain- 

 ing its food % how fix its wings on a level with or above 

 its body for skimming purposes 1 how fly in a curve ] how 

 fly with, against, or across a breeze % how project itself from 

 a rock directly into space, or how elevate itself from a level 

 surface by the laboured action of its wings 1 



The wing of the bird is elevated to a certain extent in 

 flight by the reaction of the air upon its under surface ; but 

 it is also elevated by muscular action — by the contraction of 

 the elastic ligaments, and by the body falling downwards and 

 forwards in a curve. 



That muscular action is necessary is proved by the fact 

 that the pinion is supplied with distinct elevator muscles.^ 

 It is further proved by this, that the bird can, and always 



^ Mr. Macgillivray and C. J. L. Krarup, a Danish author, state that the 

 wing is elevated by a vital force, viz. by the contraction of the pectoralis 

 mmor. This muscle, according to Krarup, acts with one-eighth the intensity 

 of the ^ec^ora^i5 ma/or (the depressor of the wing). He bases his statement 

 upon the fact that in the pigeon the pectoralis minor or elevator of the wing 

 weighs one-eighth of an ounce, whereas the pectoralis major or depressor of 

 the wing weighs seven-eighths of an ounce. It ought, however, to be borne in 

 mind that the volume of a muscle does not necessarily determine the precise 

 influence exerted by its action ; for the tendon of the muscle may be made to 

 act upon a long lever, and, under favourable conditions, for developing its 

 powers, wdiile that of another muscle may be made to act upon a short lever, 

 and, consequently, under unfavourable conditions. — On the Flight of Birds, 

 p. 30. Copenhagen, 1869. 



