July, 1893. 



THE RESPIRATION OF BIRDS. 



29 



that the contraction of the latissimus dorsi causes a movement of 

 the body and not of the wing. All possible work is put upon the 

 pectoral muscles, so that the centre of gravity may be low down, a 

 point of great importance in a flying-machine ; but though the flight 

 muscles help so largely towards respiration, of course much is done 

 by what are commonly spoken of as the respiratory muscles. The 

 levatores costarum, which I have found very highly developed in 

 the domestic pigeon, arising from the vertebrae, passing backwards 

 and attaching to the ribs some way down, tend to make the upper 

 part of the rib horizontal, thus broadening the chamber beneath. 

 The triangularis sterni, which arises from the inside of the sternum, 

 from its anterior lateral end, and attaches to the sternal ribs, works 

 towards the same object from below. The action of the external 

 intercostals is not so easy to understand. In man they raise both the 

 ribs, which they connect, and at the same time raise the breastbone ; 

 but in birds the pectoral muscles render the sternum heavy, and, 

 what is far more important than this, the weight of the body 

 hanging upon the wings during flight, and the pressure of the 

 wings inwards upon the coracoid bones and clavicles, must make 

 the breastbone practically immovable. What, then, will happen 

 when the external intercostals contract ? It must be remembered 

 that the contraction of a muscle tends to shorten the distance 

 between its two ends. The muscular band, passing obliquely 

 back and downwards, will lower the front rib to which it is 

 attached, and raise the hinder ; the loose muscular connection 

 of the vertebral column with the shoulder blades allowing some 

 depression of its anterior and a corresponding raising of its pos- 

 terior part. Moreover, the backbone of most birds that I have 

 examined bends downward easily, and through a considerable arc 

 just anterior to the ilium. The raising of the hindmost ribs which 

 articulate with the vertebrae behind the point where the bend takes 

 place, will aid the vertebral muscles in straigthening the back. The 

 accompanying diagram illustrates the working of the external 

 intercostals. 



r. = Costal rib. 



e.i. = External intercostal muscle. 



5. = Sternal rib. 



st. = Sternum. 



v.= Vertebral column. 



The contraction of the muscle 

 will lower the anterior and raise 

 the posterior rib. 



Wishing to test these conclusions by experiment, I suspended a 

 freshly-killed pigeon by its wings, and inflated the air-sacs by means 

 of a blowing tube. The backbone, a little in front of the thigh 



