II. 



The Respiration of Birds. 



JOHN HUNTER, I believe, held that birds did not draw breath 

 during flight, but merely used the air stored in the sacs. Extra- 

 ordinary as this view may seem, he was led to adopt it by what is a 

 very real difficulty, namely, that the movement of the breast in 

 breathing would seriously derange the machinery of flight. Mr. 

 Edmond Alix, in his Essai sur Vappaveil locomoteur des Oiseaux, gives 

 what I believe is the right solution of the problem, viz., that the 

 bird during flight moves its back, not its breast, up and down in the 

 act of breathing ; but he does not explain how this is effected. My 

 own investigations have led me to the conclusion that the muscular 

 movements necessary to flight themselves to a great extent bring 

 about the enlargement and contraction of the thoracic cavity without 

 which respiration cannot take place. The muscle called the latis- 

 simus dorsi, which arises from the vertebrae, and, after passing over the 

 scapula, is inserted in the humerus, raises the backbone during the 

 down stroke of the wing, bringing the bird's body into the position 

 which is required for horizontal flight ; and not only is the backbone 

 raised and brought to, or nearly to, the horizontal, but at the same 

 time the ribs are straightened, and this causes an expansion of the 

 air-sacs. The effect of this raising of the back may be seen if a bird 

 is suspended by its backbone, when the weight of the sternum and 

 the breast muscles hanging upon the ribs causes them to straighten 

 out at the junction of their costal and sternal portions. The muscles, 

 also, which pass downwards to the ribs from the posterior ends of the 

 scapulas, will help towards the same result, and, besides that, will 

 broaden the roof of the thoracic cage, the upper part of the rib tending 

 to become more horizontal when the lower part is raised. The 

 action of these last-named muscles is assisted by the movement of the 

 scapula. It will be found that during the down stroke, when the 

 front margin of the wing is lower than the hind margin, so that the 

 undersurface of the wing looks downwards and backwards, the 

 anterior end of the scapula is depressed very slightly, and this causes 

 a considerable raising of the posterior end. Thus in birds, as in many 

 crustaceans, progression itself aids greatly the process of respiration. 

 Another point worth noticing is that in this the great pectoral plays 

 an important part, for it is due to the great strength of this muscle 



