364 COMPARATIVE ANATOMY 



gradually formed (secondary and tertiary bronchi, &c.). The ends 

 of these branches are swollen, forming crypts known as infundibula, 

 which are made up of a number of alveoli and are surrounded by 

 blood-capillaries, through the thin walls of which the interchange 

 of respiratory gases takes place (Figs. 269 and 270). The bronchi 



o 



FIG. 270. DIAGRAM OF THE EMBRYONIC HUMAN LUNG, :: 50. 



(After W. His.) 



Af>, pulmonary artery ; Ib, pulmonary vesicle undergoing division ; Ir, air- 

 passage ; M, middle lobe of the lung ; O, right anterior upper lobe with 

 its " eparterial" bronchus; O 1 , left anterior lobe with its " hyparterial " 

 bronchus ; sp, oesophagus ; F, T n , right and left posterior lobe. 



are lined by ciliated epithelium, the infundibula and alveoli by 

 pavement epithelium. 



Thus a great increase in the respiratory surface is gradually 

 produced in the ascending series of Vertebrates and in the 

 individual development of the higher forms, in which the lungs 

 may also become secondarily divided up into lobes. 



In the following account the air-tubes will be dealt with 

 separately from the lungs proper. 



Air-Tubes and Larynx. 



The walls of the air-tubes may consist, in addition to their 

 lining of ciliated epithelium, of connective tissue and elastic and 

 muscular fibres only, but as a general rule cartilaginous elements 

 are also formed, and these serve to keep the tubes permanently 

 open. From the Amphibia onwards the most anterior of these 

 cartilages, which support the larynx, become differentiated to form 

 a frame on which are stretched the structures by means of which 

 the voice is produced the vocal cords : these cartilages are acted 

 upon by muscles. The relative length of the windpipe as a 

 general rule corresponds with that of the neck. 



In certain Fishes there may be a more or less complicated 

 arrangement of the muscles derived from those of the gill-arches 



