65 



Hue of flight is reached, and at the end of the pulse the point readied is- 

 abont on a level with the position before the pnlse commenced. This may 

 of course be, and very likely is, merely the effect of propulsion and similar to 

 the flight of a bullet through the air. But may it not also be that the air 

 sacs are filled for rising, and as the air becomes exhausted the bird drops 

 only to rise again when the air sacs are re-filled. This is of course taking it 

 that the sacs are in constant use. This would embody Theory No. 5, and 

 would also be more or less applicable to my own theory, as the up and 

 down strokes of the flight are far more noticeable, in wind)' weather, in 

 almost all birds. 



Now how does he argue when he takes the Ostrich, or even the 

 barn door fowl, who do very little in the way of flying. One would 

 expect, believing Mr. Miiller, that as in adapting themselves to flight they 

 put off or lost all superfluous material, so when they give up the practical 

 everyday use of the wings for flight, they would fill their air spaces with 

 something more substantial than air. To poultry keepers this would be a 

 benefit, but facts are facts. The Ostrich may still require to draw upon his 

 reservoirs, if running against a head wind, and the barn door fowl might be 

 called upon to keep running without the chance of an external inspiration. 

 But the empty sacs in " their emptiness " are really of no benefit to them. 



Will some of the readers of Bird Notes let me have their theories on 

 this matter. Now that the problem of aerial navigation is before the public, 

 main' lessons could I fancy be learnt, in construction, by studying the 

 internal construction of our feathered friends. 



Mr. Mullet's arguments to disprove the respiratory functions of air 

 sacs from The Field, March 2SU1. " Let us suppose that during inspiration 

 the lungs and air sacs are filled with fresh air; that which reaches the lungs- 

 would then give off a portion of its oxygen, that in the air sacs would not. 

 During the expiration the air from the lungs would be driven out through 

 the bronchi into the trachea, and the air from the sacs would pass into the 

 lungs. There the latter air would be deprived of a portion of its oxygeiu 

 During the next inspiration this air (poor in oxygen) would again pass from 

 the lungs into the air sacs (I say here there are two inspirations to one 

 expiration. — I. M.) During the next inspiration (sic.) the same air would 

 then again fill the lungs, and these would then always contain air poor in 

 oxygen, and therefore ill adapted for respiratory purposes. 



The writer of the article to The Field adds " Assuming the premises 

 to be correct the argument appears to be conclusive." 



[We publish the above with some misgivings as to its general acceptance, but the 

 subject is of much interest and further discussion is invited. Kr>.l 



I cannot believe Nature ever did anything purposelessly or accidentally, let alone 

 gave parts that were of no use or had no function. I believe the air sacs of birds which 

 communicate with the lungs on the one hand, and the interior of most of their bones on 

 the other, have, in conjunction with the lungs and bones, many functions ; the principal of 

 which are respiration, locomotion by diminishing the weight of the body, and, by the 



