No. I.] THE STRUCTURE OF THE LUNG. 1 75 



sections which are cut in such a plane that a vestibule is 

 divided at right angles. In such sections we find the bundles 

 of muscle fibers, cut transversely, lying on either side of the 

 divided vestibule, but they do not in the least enter into the 

 formation of the walls of the cavity of the atrium. 



The air-sac passages connecting an atrium with its air-sacs 

 are somewhat smaller than the vestibules, having an average 

 diameter of 0.143 mm. (Fig. 21). In sections the air-sac pas- 

 sages can always be distinguished from the vestibules by the 

 absence of smooth muscle fibres. 



The air-sacs present a great diversity of forms; they are 

 very irregular and adapt themselves to the space they have to 

 occupy. As seen in sections of the lung, they vary in diameter 

 from 0.313 mm. to 0.51 1 mm. In Fig. 21 a corrosion model 

 of an atrium with a single air-sac attached is shown. The 

 irregularity of form is well shown in Fig. 22, which is a single 

 air-sac removed from its atrium ; it also shows a deep cleft 

 nearly dividing it in half. The irregular contour of one air-sac 

 fits into corresponding irregularities of the adjoining air-sacs. 

 The walls of the air-sacs are quite thin and are made up of 

 the capillary network of the blood-vessels, elastic tissue and 

 reticulum. In sections lying parallel with, or perpendicular to, 

 the pleura, the large irregular-shaped openings bounded by 

 thin walls are the air-sacs. The small openings grouped 

 about them are the air-cells. 



The air-cells are about one fourth the size of the air-sacs; 

 their walls are thin, and have the same structure as those of 

 the air-sacs. There are three varieties of air-cells: those 

 arising from the bronchus, those from the atrium, and those 

 from the air-sac. Those arising from the air-sac are the more 

 numerous, and are the most pronounced on its distal end. The 

 air-cells arising from the atria and air-sacs have the same aver- 

 age diameter of 0.113 mm., while those air-cells which are 

 found on the bronchus are smaller, having an average diameter 

 of 0.047 JTim. It is not uncommon to find in sections made per- 

 pendicular to the pleura that the plane of the section is such 

 that it divides longitudinally a vestibule opening into an atrium. 

 In rare instances it may pass through the vestibule, atrium, 



