THE DIGESTIVE AND RESPIRATORY SYSTEM 315 



physiological efficiency through a repeated subdivision of the 

 interior. This results in the production of small chambers, 

 commonly known as " air-cells," more properly alveoli, which 

 are connected with the bronchi by means of numerous smaller 

 branches. The walls of these alveoli are covered with a net- 

 work of capillaries, thus making them the ultimate organs 

 of respiration to which all other parts are accessory. Pri- 

 marily there are no cartilages in the lungs themselves, but 

 in reptiles they may be seen to develop along the course of 



a 



CH 



FIG. 88. Larynx of Echidna- (monotreme). [After GCEPPERT.] 



(a) Ventral, (b) Lateral. 



St. H, stylo-hyal; EH, epi-hyal; CH, cerato-hyal; BH, basi-hyal; Th. H, thyro- 

 hyal; Thy, I, first thyreoid cartilage; Thy, z, second thyreoid cartilage. 



the bronchi and invade the lungs; in mammals the smaller 

 bronchial tubes are similarly equipped, almost as far as the 

 ultimate branches, although in the course downwards the 

 rings become less complete and are finally reduced to irregu- 

 lar pieces lying in the sides of the tubes. The smallest tubules, 

 which are without cartilaginous pieces, are termed bronchioli. 

 In birds and in many mammals the lungs are subdivided 

 by grooves into lobes, but in other cases the grooves are shal- 

 low, and the lobes become hardly more than slight protuber- 



