THE MUSCULAR SYSTEM. 203 
so 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 raise 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, indeed, 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 ealled 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. For this 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 +, 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 a 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 wing, and by rotating them 
allow the air to pass between them each time the wing is elevated 
in flight. 

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