116 



ANIMAL LOCOMOTION. 



and bird so constructed is stronger, weight for weight, than 

 one composed of solid matter, we may dismiss the subject; 

 flight being, as I shall endeavour to show by-and-by, not so 

 much a question of levity as one of weight and power intelli- 

 gently directed, upon properly constructed flying surfaces. 



The bodies of insects, bats, and birds are constructed on 

 strictly mechanical principles, — lightness, strength, and dura- 

 bility of frame being combined with power, rapidity, and 

 precision of action. The cylindrical method of construction 

 is in them carried to an extreme, the bodies and legs of 

 insects displaying numerous unoccupied spaces, while the 

 muscles and solid parts are tunnelled by innumerable air- 

 tubes, which communicate with the surrounding medium by 

 a series of apertures termed spiracles. 



A somewhat similar disposition of parts is met with in 

 birds, these being in many cases furnished not only with 

 hollow bones, but also (especially the aquatic ones) with a 

 liberal supply of air- sacs. They are likewise provided with a 

 dense covering of feathers or down, which adds greatly to 

 their bulk without materially increasing their weight. Their 

 bodies, moreover, in not a few instances, particularly in birds 

 of prey, are more or less flattened. The air-sacs are well 

 seen in the swan, goose, and duck ; and I have on several 

 occasions minutely examined them with a view to determine 

 their extent and function. In two of the specimens which I 

 injected, the material employed had found its way not only 

 into those usually described, but also into others which ramify 

 in the substance of the muscles, particularly the pectorals. 

 No satisfactory explanation of the purpose served by these 

 air-sacs has, I regret to say, been yet tendered. According 

 to Sappey,^ who has devoted a large share of attention to the 

 subject, they consist of a membrane which is neither serous 

 nor mucous, but partly the one and partly the other ; and as 

 blood-vessels in considerable numbers, as my preparations 



1 Sappey enumercates fifteen air- sacs, — the thoracic, situated at the lower 

 part of the neck, behind the sternum ; ttvo cervical, which run the whole 

 length of the neck to the head, Avhich they supply with air ; ttoo imirs of 

 anterior, and two pairs of posterior diajphragmatic ; and two ;pairs of abdo- 

 minal. 



