ECONOMY OF THE BODY 127 



is a complex and intimate relationsiiip between air-sacs and 

 syrinx. 



The short bronchi divide up in a pecuHar fashion to 

 form the lung. A main stem goes right through as the 

 mesobronchus and leads into the abdominal air-sac. Near 

 its end it gives off a branch which leads into the post-thoracic 

 air-sac. But soon after it enters the lung the main stem 

 shows a dilatation or vestibule, and this gives off a number 

 (four in the pigeon) of secondary bronchi or ectobronchia. 

 These lead into other air-sacs — an interclavicular, two 

 cervicals, and two anterior thoracics. 



From the mesobronchus and the ectobronchia there 

 arise tertiary bronchi, some of which unite with one another. 

 They give off minute canaliculi, through the walls of which 

 there is gaseous interchange with the blood. 



There is a median vertical septum of connective tissue 

 between the two lungs, and this is continuous with a fibrous 

 connective tissue " pleural membrane," which covers the 

 ventral surface of the lungs and binds them at their edge to 

 the thoracic wall. The dorsal attachment of the lungs to 

 the ribs is in part muscular. This envelope of the lungs 

 bears some resemblance to the muscular diaphragm of 

 mammals, but there is no homology. It does not shut off 

 the " chest " from the visceral cavity, for the air-sacs go 

 through it ; moreover, the pericardial space in which the 

 heart Hes is quite outside it. What has been called " the 

 oblique septum " in birds simply consists of the walls of 

 the air-sacs in so far as these protrude into the body-cavity 

 and are not adherent to the body- wall. 



The air-sacs are Hke soap-bubbles ; they are ciliated 

 internally ; they have few blood-vessels on their walls. 

 As they are full of hot air, they must make the bird a little 

 more buoyant, but so Uttle that the lightening is counter- 

 acted by a few mouthfuls of solid food. Their functions 

 are not in this direction. They considerably increase the 

 content of air which a bird has in its respiratory system, 

 and this is useful in the prolonged immersion of diving 

 birds and in enthusiastic song. But there is a further 



