386 



COMPARATIVE ANATOMY 



a 



a 



between the muscles, beneath the skin, and even into most 

 of the bones. The latter are thus rendered pneumatic, and conse- 

 quently the buoyancy of the body is increased. The pneumati- 

 city of the bones is not, however, an essential peculiarity connected 

 with flight, for in many Birds which are extremely good fliers 

 (e.g. Larus, Sterna) the bones are hardly if at all pneumatic, while 



in the cursorial Ratitse, on the other 

 hand, they are markedly so. 1 



The air-sacs, though not serving 

 to increase the actual respiratory 

 surface, 2 must be looked upon as an 

 integral part of the respiratory ap- 

 paratus : by their means, a greater 

 quantity of air can rapidly pass in 

 and out through the lungs when the 

 body-cavity is expanded and con- 

 tracted during inspiration and expira- 

 tion respectively, especially through 

 the larger bronchi ; consequently 

 there is less need for the expansion 

 of the lung-parenchyma. Moreover, 

 as part of the inspired air passes 

 directly into the air-sacs and their 

 prolongations, the absorption of oxy- 

 gen can take place during expiration 

 as well as inspiration. The aeration 

 of the blood is thus very perfect and 

 its temperature correspondingly high. 

 Rhythmical respiratory move- 

 ments take place when the Bird is 

 at rest, the sternum being alternately 

 raised and lowered. But during 

 flight, when the weight of the body 

 is supported by the wings, the 

 sternum, as well as the coracoid and 

 ribs, are relatively immovable, and 

 inspiration and expiration are effected 

 by the raising and lowering of the 

 wings, which may take place from three to thirteen times in the 



1 The bones of Archsopteryx were solid, and those of the recently extinct 

 Moa of New Zealand were much less pneumatic than in existing Ratitse. 



Pneumaticity of the bones is not a special peculiarity of Birds ; it occurred 

 amongst the gigantic fossil Dinosaurians, and the skull of Crocodiles is also 

 pneumatic. Amongst Mammals, frontal, maxillary, and splienoidal sinuses 

 are present in Anthropoids, Ungulates, Elephants, and Marsupials for instance : 

 and all these communicate with one another, and also with the tympanic 

 cavity. They are in many cases developed in order to give a greater surface 

 for the attachment of muscles, and also to effect a saving of material and a 

 lightening of the skull. 



2 The walls of the air-sacs are supplied with blood by small nutrient arteries 

 arising from che aorta, the veins communicating with the postcaval. 



FIG. 292. DIAGRAM OF THE AR- 

 RANGEMENT OF THE BRONCHI IN 

 MAMMALS. From the ventral 

 side. 



A, pulmonary artery; a, a, 

 "eparterial" bronchus of either 



bronchi ; V, pulmonary vein. 



