i8 93 . THE AIR-SACS OF BIRDS. 



349 



posture takes only from 13-15 breaths in the same time. During flight 

 a bird's breathing must be far more rapid, and it is impossible to avoid 

 the conclusion that its temperature is kept equable by this means. 



There remains the difficult question of the aeration of bones. It 

 is possible that this also may be of some slight use in the regulation 

 of temperature ; but if this is so, it can only be a very secondary 

 object, since the air cannot be expelled from them at will. Before 

 proceeding to discuss the problems connected with pneumaticity, I 

 will briefly set down the main facts. 



(1.) Many small birds that are first-rate flyers have either 

 marrow in all the large bones, or else in all except the humerus. 



(2.) Most of the big strong-flying birds have a great deal of 

 aeration. 



(3.) The hornbills, which, according to good observers, are very 

 poor flyers, are as pneumatic as any birds, or, perhaps, more so than 

 any. 



(4.) Birds which dive have solid bones or only the humerus 

 aerated. 



(5.) Birds which spend much of their time in the water without 

 diving have, at least in all the cases which I have been able to 

 investigate, nearly all the bones solid. 



(6.) There are great differences between nearly-related species, 

 e.g., the gannet has an extraordinary amount of aeration, while its near 

 ally, the cormorant, has only the humerus pneumatic. The hornbill 

 is not very distantly related to the swift, which is singularly deficient 

 in aeration. 



(7.) The bones of birds that are highly pneumatic are, relatively 

 to their length, larger in girth than those of birds in which little 

 aeration is found. 



To find one's way through these facts is not easy. But one 

 point must strike anyone. Great pneumaticity might be an incon- 

 venience to a diver. As it is, he can regulate the amount of his body 

 that appears above the water, sometimes sinking till no more than 

 his head is visible. This, no doubt, he effects by driving the air from 

 his sacs. But aerated bones would certainly not help him to vary 

 his specific gravity, and they would make it more difficult for him to 

 swim under water. Probably, the marrow in the bones serves 

 a very important physiological purpose. Divers are frequently 

 exposed to great cold when in the water. They are protected against 

 this by a peculiarly thick coat of feathers, and by a deep layer of fat 

 beneath the skin ; and I cannot help thinking that the marrow also 

 helps to maintain their warmth. It is held that in it a large 

 proportion of the red blood corpuscles are generated, and 

 unless they are very thick in the blood, a high temperature cannot 

 be maintained. But if the marrow is a factory of red corpuscles, 

 what substitute for this have birds whose bones are marrowless ? 

 Though, as a rule, exposed to less cold than water-birds, they have a 



