No. I.] THE STRUCTURE OF THE LUNG. 1 77 



cells. As far as I have studied the subject, there is no sec- 

 ondary communication between adjacent air passages in any- 

 animal, excepting the bird. 



The Blood-Vessels. 



Besides the pulmonary artery and vein, we have to consider 

 the bronchial vessels. I shall not consider the grosser vessels, 

 for they have been described so often and can be demonstrated 

 so easily by means of corrosion. The pulmonary artery follows 

 the bronchus throughout its entire length, and when it reaches 

 the last forked division of the bronchus it penetrates the lobule 

 until it reaches a point beyond the terminal bronchus. Fig. 1 3 

 shows the relation of the artery to the bronchus just before the 

 lobule is reached, while Fig. 15 includes also the vein. In all 

 sections of the lung the bronchus and artery are side by side, 

 while the vein is at all times as far away as possible from them. 



After the terminal bronchus is passed, the artery divides 

 quite abruptly into as many branches as there are atria, as 

 shown in Fig. 25. From the atrial arteries the terminal arteries 

 arise and supply the air-sacs. It is extremely difficult to demon- 

 strate the real relation from the figures, as the relation is at 

 once obscured by the many air-sacs. Cut 8, however, is an 

 exact diagram from the corrosion specimen, reduced to a 

 single plane. Each air-sac receives a single artery on its 

 central side, i.e. the side over the center of the lobule, 

 which at once breaks up into a rich capillary network, as 

 shown in Fig. 16. 



We have already seen that there are three kinds of air-cells, 

 those arising from the terminal bronchus, those from the atria, 

 and those from the air-sacs. Corresponding with this subdi- 

 vision we also have three kinds of terminal arteries; those for 

 the air-sacs, those for the atria and those for the air-cells arising 

 from the terminal bronchus. This arrangement is of course 

 what would be expected, for no doubt all the above subdivisions 

 function alike and all must have an equal arterial supply. It 

 does not seem as if any portion of the lobule is especially 

 favored, because the small branches to the atria are smaller 



