vi FORM AND FUNCTION 89 



breastbone, from which spring the great muscles which 

 lower and raise the wing. I f the 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 to the 

 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 vertebrae just behind the neck and also 

 from vertebrae 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, 



