vI FORM AND FUNCTION 89 
breastbone, from which spring the great muscles which 
lower and raise the wing. Ifthe breast were perpetually 
“moving up and down, a strong stroke, such as flight 
requires, would be impossible. M. Edmond Alix gives 
what I believe is the right explanation—viz., that a bird 
breathes during flight, by moving not its breast, but 
its backbone. But he does not explain how this is 
effected. After some investigation I have come tothe 
conclusion that the muscular movements necessary to 
flight themselves lend material assistance in the process 
of breathing. I have here, therefore, to anticipate 
some important points which properly belong to the 
chapter on flight. There is a very broad sheet of 
muscle, called the Latissimus Dorsi, which arises 
from the vertebra just behind the neck and also 
from vertebre further back, sometimes even from the 
pelvis. It attaches to the shoulder bone. When the 
wing is lowered in flight, this muscle contracts and 
hauls upon the wings, which resist its action since 
stronger muscles are pulling them in a different 
direction. Since the wings will not give, the body is 
lifted towards them, attaining nearly to a horizontal 
position. Were it not for this muscle, it would hang 
nearly straight down, as you may see by taking a 
dead bird and holding it by its outstretched wings. 
The Latissimus Dorsi not only keeps the body nearly 
horizontal, but expands the air-sacks. For when the 
back is raised, the weight of the breast muscles and 
the intestines hanging on the ribs will straighten them 
out, and so the sacks which lie close under the back 
will be distended. This may easily be seen if you 
grip the backbone of a dead bird with a pair of pincers, 
