THE MirsCULAE SYSTEM. 203 



BO the muscles of flight are the most voluminous — those muscles, 

 that is, which raise and depress the wing. 



In ourselves the muscles which draw back and Taise the arm 

 are situated on the dorsal surface, while those which move it 

 forwards are on the breast. In Birds, however,- as almost 

 everyone must have noticed, there is very little flesh on the 

 back, and, iudeed, both these sets of muscles are on the breast, 

 which is the most convenient arrangement with respect to the 

 centre of gravity, though it necessitates a very special modifi- 

 cation. They are called pectoral muscles. It is said that 

 these muscles in the Swift weigh more than all the other muscles 

 of its body taken together. 



The largest muscle of a Bird is that on the surface of the 

 breast, which is called the great pectoral. It takes origin from 

 the sides of the keel and some other parts of the sternum, and 

 is inserted into the inner side of the crest* on the anterior 

 surface of the head of the humerus, and by its action depresses 

 the wing. 



Beneath this great muscle there is another, called the second 

 •pectoral — or supracoracoideus — which has an extensive origin 

 from the sternum, and is a powerful muscle devoted to antago- 

 nizing the action of the great pectoral — that is, it raises the 

 wing. J"or tliis purpose there is the special modification (to 

 which we before referred) to compensate for the ventral situation 

 of the muscle itself. The fibres of the second pectoral are 

 inserted into a tendon which passes through the interspace 

 left at the junction of the coracoid, scapula, and clavicle f, the 

 bony margin of which serves it as a pulley. Passing round the 

 coracoid it is inserted into the upper end of the crest of the 

 humerus, and is thus enabled to act as an elevator of it and 

 therefore of the whole wing. This muscle is particularly strong 

 . in the Penguin and other diving birds. 



There is a third pectoral, a small muscle, which arises from 

 the sternum and often from ,the coracoid, and thence passes 

 directly to the humerus, which it depresses. There is some- 

 times also sb fourth pectoral, which is similar to the third one. 



The skin of birds contains many very small muscles which 

 act upon the individual feathers. Most important are those 

 which act on the remiges of the vidng, and by rotating them 

 allow the air to pass between them each time the wing is elevated 

 in flight. 



* See ante, p. 190. t^ee ante, p. 190. 



