LUNGS 



387 



second. Thus during flight the air passes in and out without any 

 special respiratory movements being made, and the bird can travel 

 rapidly and continuously through the air without getting out of 

 breath. 



A further importance of the air-sacs consists in the resulting 

 enlargement of the anterior part of the body surrounded by the 

 pectoral arch. An extended development of the skeleton can thus 

 take place, giving an increase of surface for muscular attachment 

 without any considerable increase in weight. Everything, in fact, 

 combines to establish an organ of flight with a large wing-surface 

 and increased muscular power. 



Mammals. Though not directly derived from the reptilian 

 type of lung, a certain parallelism can be recognised between the 

 lung of Reptiles and that of trie Echidna as regards, for instance, 

 the presence of large air-spaces ; but the resemblance here is 

 probably only of a secondary nature. 



The main bronchus extends 

 throughout the length of the lung 

 and gives off a double row of second- 

 ary bronchi on its dorsal and ventral 

 aspects respectively, the components 

 of the ventral system being larger 

 than those of the dorsal. 1 



The morphological importance of 

 the lobes into which the lungs are 

 usually more or less divided (Fig. 

 293) is secondary to that of the 

 branching of the bronchi, and does 

 not essentially affect the latter: the 

 furrows between the lobes frequently 

 disappear to a greater or less extent. 



The right lung possesses in many FIG. 293. LUNG OF MAN. 

 cases an accessory lobe anteriorly, 

 and another posteriorly, the former S, sulcus for the subclavian artery ; 

 in connection with an apical bronchus t^in^^cor^a ^T^l ^lobe's 

 and the latter (" azygos lobe ") with O f t he right, and '2a, 3a, of the 

 an accessory bronchus arising ven- left lung, 

 trally from the main bronchus, and 



this accessory bronchus may be present even if the azygos lobe is 

 undifferentiated. 



The cartilages of the bronchi become more and more sparse 



1 The most anterior of the bronchi may arise from the main bronchus or 

 even from the trachea anteriorly to the point at which the pulmonary artery 

 crosses the main bronchus, and thus has been distinguished as the "eparterial 

 bronchus" from the others, or " hyparterial bronchi," which arise posteriorly to 

 this point (Fig. 292). As a general rule, an " eparterial bronchus" is present on 

 the right side only, and as the nature and meaning of this asymmetry are not 

 clear, it is better to speak of the anterior secondary bronchi, whether " epar- 

 terial" or "hyparterial," merely as apical bronchi. 



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