XII AVES 431 



bird brought about by muscular contraction^ that of inspiration simply 

 by the passive elasticity of the skeleton. 



The lung itself in the restricted sense undergoes during its develop- 

 ment great complication in its minute structure. In all other vertebrates 

 the respiratory surfaces of the lung are in the form of blindly ending 

 pockets. In the bird on the other hand the originally blindly ending 

 pockets become joined up by their tips to form continuous channels. 

 In the fully developed lung these are very complicated, the main intra- 

 pulmonary bronchus giving off side branches, these in turn giving off 

 numerous parabronchi arranged parallel to the surface of the lung, and 

 the parabronchi communicating with an intricate network of exceedingly 

 fine air-capillaries in the closest possible relation with the blood-capillaries 

 — the two sets of capillaries with their very thin dividing walls con- 

 stituting a quite unrivalled mechanism for respiratory exchange between 

 the air and the blood. While the main fact is clear enough that the 

 bellows-like arrangement of the air-sacs provides for the passage of air 

 inwards and outwards through the cavities of the lung, it is not yet 

 definitely worked out in detail by what arrangements the air-stream 

 is deflected through the air-capillaries. 



The lung of the bird, apart from the air-sacs, is of relatively small 

 size and is moulded closely to the inner surface of the body-wall. Its 

 connexion with the pharynx is by way of a long trachea which forks at 

 its hinder end into the two bronchi. Both trachea and bronchi are 

 strengthened by cartilaginous rings. At the pharyngeal end these are 

 somewhat modified to form a larynx but it is eminently characteristic 

 of the birds that the organ of voice is not the larynx but a special organ, 

 the syrinx. This is formed by modifications, differing in degree in 

 different types of bird, of the tracheal tube in the region where it divides 

 into the two bronchi. Parts of its wall are reduced to thin membranes, 

 the tension of which can be varied by special muscles, and the voice is 

 produced by these being thrown into vibration by the air-current. 



The most conspicuous function of the air-sacs has already been dealt 

 with but there is also an important accessory function. During the 

 expansion of the air-sacs in the bird embryo their walls come at numerous 

 points into contact with the bones of the skeleton and where this happens 

 the bony tissue is absorbed and the air-sac wall burrows its way along 

 the axis of the bone, taking the place of the marrow. In this way the 

 bones of the bird are rendered pneumatic and the available amount of 

 bony substance is used to the greatest advantage in the construction of 

 large hollow, instead of small solid, bones. This fact that the bones of 

 the skeleton are traversed by air-spaces in communication with the lung 



