232 Cambridge Philosophical Society. 



out of the air-cells and bronchial tubes of various orders is difficult 

 to describe, especially without diagrams. 



The respiration of birds, even when in repose, has been shown to 

 be much more active than that of mammals. But in order that 

 birds may be equal to the enormous exertion required of them for 

 sustaining themselves in the air for considerable periods of time, very 

 ample provision must be made for respiration. If therefore the lungs 

 were constructed after the mammalian type, they would require to 

 be very large, and powerful muscles must have been provided for the 

 respiratory movements. But this would add unduly to the weight of 

 the body. The lungs therefore are small, very porous, and light ; 

 yet nevertheless their efficiency is ensured by a more minute division 

 of the capillaries, and a more complete exposure of these to the 

 action of the air supplied so abundantly from the capacious air-sacs. 

 In short, more perfect localized instruments of respiration cannot be 

 conceived. 



Our great physiologist, John Hunter, believed it impossible that 

 the ribs and sternum of a bird could move while the powerful pec- 

 toral muscles are engaged in r v flight. He therefore thought that the 

 air-sacs of birds might be intended to act as reservoirs of air to be 

 used in respiration during flight. These sacs, however, do not hold 

 enough air to support the respiration of a bird for two minutes ; for 

 in that time, if the trachea of a bird be tied, it dies ; yet many birds 

 continue on the wing for hours together. Sappey has endeavoured to 

 explain the difficulty which occurred to Hunter by pointing out that the 

 great pectoral muscles of birds arise exclusively from the sternum, and 

 not at all from the ribs, as they do in mammals. But this explana- 

 tion only removes a part of the difficulty; for the ribs are so articulated 

 with the sternum, that they cannot move unless the sternum moves 

 also. Now the sternum in respiration moves at its articulations 

 with the two coracoid bones, these bones being fixed in regard to 

 the sternum and humerus in the movements of flight. It might 

 seem, therefore, that when the pectoral muscles contract, the ster- 

 num would be drawn powerfully upwards as the wings are drawn 

 downwards, and so the sternum and ribs fixed. But this is not so ; 

 for the fibres of these muscles converge towards and pass over the 

 coracoid bones on their way to be inserted into the ridge of the 

 humerus, and they act in the direction of the axis of the coracoid ; 

 so that they only draw the sternum and coracoid together more 

 closely, and do not tend to flex these bones on one another. The 

 common inspiratory muscles are therefore free to act, whether the 

 pectorals are in action or not. To be more exact, the line of action 

 of the great pectoral muscle lies a little below the coracoid bone, and 

 parallel to its axis. Hence, in contracting, the muscle will tend to 

 depress the sternum, and so assist the inspiratory muscles, and ren- 

 der inspiration deeper in flight than when the wings are closed. 



The author gave a mathematical as well as an experimental proof 

 that the external intercostal muscles raise both the ribs to which 

 they are attached, and that the internal intercostals depress both ribs. 



