72 THE FLIGHT OF BIRDS 



Sometimes, in order to carry out a movement, two 

 muscles have to antagonise 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 

 fibrillse, and are buried in a granular material known 

 as sarcoplasm. Muscles which are capable of long- 



