72 THE FLIGHT OF BIRDS 
Sometimes, in order to carry out a movement, two 
muscles have to antaggmise one another. When a 
bird wishes to check his speed suddenly, he lets his 
body hang downward, sometimes almost vertically, 
and holds his wings so that their under-surface faces 
in the direction of his flight. Now, if the Depressor 
were alone holding the wing, it would lower the front 
relatively to the back, and the resistance of the 
air would reinforce its action, making the wing turn 
its under-surface downward as in ordinary flight, 
whereas for checking speed it must face to the front. 
To orient it thus, by holding fast the front margin 
of the upper face of the humerus, is the work of 
the Elevator. The antagonising of one muscle by 
another is, of course, constantly going on. Were 
it not for this, with its steadying effect, no movement 
could be carried out with precision, no attitude could 
be maintained. There are, of course, other muscles 
which contribute to the working of the bird’s wing, 
and, notably, there is a third pectoral, the office of 
which is to draw the wing back. But it is no part 
of this short treatise to describe the work of minor 
muscles. 
I have already alluded to the different qualities 
of muscle found in birds. Muscle of the highest 
quality—capable of an enormous number of very 
rapid contractions in rapid succession—consists of 
fibres in which the fine striation in the direction of 
the pull of the muscle is much obscured by irregular, 
highly granulated cross ridges. In colour it is a rich 
red. The fibres consist of small, contractible 
fibrillee, and are buried in a granular material known 
as sarcoplasm. Muscles which are capable of long- 
