ECONOMY OF THE BODY 129 



air-sacs have no positive function, but are rather to be 

 thought of as empty spaces whose value lies in their empti- 

 ness. They facilitate the movements of the organs in the 

 thorax, especially the heart. This view over-emphasises 

 one aspect. 



A good account of the detailed structure of the bird's 

 lung is given by A. Juillet (1912), who emphasises the 

 important fact that the branches of the bronchi in the lung 

 never end in culs-de-sac, but are all in inter-communication, 

 forming circuits which can be traversed by pure air, from 

 one end or the other, according as the air comes from the 

 air-sacs or from the trachea. The lung thus consists of a 

 labyrinth of air-containing branches inter-penetrated by a 

 labyrinth of blood-capillaries. Every blood capillary is 

 surrounded on all sides by air. Thus the bird's lung has 

 a structure very different from that of the mammal's lung. 



The joining of one bronchial branch with another is 

 important ; it leads to the formation of " bronchial circuits," 

 making the lung like a labyrinth. According to W. A. Locy 

 and O. Larsell (19 16), much importance should be attached 

 to the " recurrent bronchi," recognised by Schulze and 

 Juillet, which spring from the air-sacs and grow back into 

 the lungs, where they establish numerous connections with 

 the bronchial branches. So that the air-sacs are not to be 

 thought of altogether, at least, as terminal sacs ; they may 

 also be regarded as expanded reservoirs on the course of the 

 bronchial circuits. 



There are several points of great interest in regard to 

 the development of the bird's lung. Like every other 

 lung it arises as a ventral pouch from the anterior end of 

 the food-canal, and is therefore lined by endoderm. But 

 as it grows out from the food-canal it carries with it an 

 ensheathment of mesoderm, which is most strongly developed 

 on the ventral aspect of the lung, exposed to the body- 

 cavity. To put it more precisely, according to the researches 

 of W. A. Locy and Olof Larsell (19 16), the endodermic 

 pouch lies in a layer of mesenchyme that is bordered on the 

 surface towards the body-cavity by a well-defined layer of 



K 



