THE MACHINERY OF FLIGHT 73 
continued exertion are rich in sarcoplasm, which 
would seem somehow to supply food to the muscle 
proper, to the contractile fibrille ; at any rate this 
is a reasonable’ theory that explains the facts.* 
Now the muscle which in strong-flying birds is 
reddest and, in section, most ridged and granulated, 
is the Great Pectoral that lowers the wing. This 
is a muscle of great size, as I have already pointed 
out, and its output of unflagging energy is truly 
astonishing. The Elevator muscle is paler and 
smoother, and almost certainly of far inferior quality. 
Much less is demanded of it. As soon as the bird 
has got up pace, the resistance of the air lifts the 
wing, and the Elevator has little to do. In fact the 
great Depressor may claim to lift the wing, since to 
it is due the velocity which relieves the Elevator of 
its work. In the Blackcock, the Red Grouse and 
the Ptarmigan, the paleness of the Elevator forms 
a striking contrast against the rich red of the great 
muscle under which it lies. Among birds whose 
flight muscles I have examined, the Guillemot is 
the only one whose Elevator is as rich a red and 
almost as stringy as the Depressor. Is this explained 
by the fact that the bird uses his wings in swimming ? 
Has he to raise his wings in the water by muscular 
effort ? And is this due to the fact that when he 
swims he has less pace than when he flies ? In the 
Chicken, since it seldom uses its wings, all the 
Pectoral muscles are very pale and smooth. The 
wild Jungle Fowl, from which our domestic bird 
springs, uses its legs far more than its wings, and, I 
believe, domestication has not much altered the 
* See Starling’s Physiology on quality of muscle (pp. 87, 88). 
