No. I.] THE STRUCTURE OF THE LUNG. 1 67 



power employed in reconstruction was one hundred, and each 

 section was drawn on a wax plate two millimeters thick. As 

 long as the terminal bronchus was in the section it was quite 

 easy to locate all the air-cells from section to section. Beyond 

 the terminal bronchus the location of the blood-vessels and the 

 shape of the air-sacs served as guides. In this way all the air- 

 cells communicating with one bronchus were drawn on wax 

 plates, and the portions representing the air-spaces carefully cut 

 out. The frame-work left, when piled up, gave an exact model 

 of the air-sacs, and the pieces piled gave a "corrosion" of the 

 same. The models were now cut in various directions in order 

 to study the relation of the air-sacs to the terminal bronchus. 



Throughout my study the relation of the blood-vessels to 

 one another as well as to the air-passages has been constantly 

 kept in mind, and it was found that they are distributed ac- 

 cording to rule throughout the lung, and that the terminations 

 of the bronchi are marked by sacs which have an arterial and a 

 venous side. These I have termed the air-sacs. They are in 

 turn studded with the air-cells, some of which are covered with 

 arterial and some with venous capillaries. That the blood- 

 vessels are related to the component parts of complex glands 

 in this manner is by no means peculiar to the lungs, for it 

 has been shown that the same exists in all organs in which 

 the blood-vessels have been studied carefully.^ 



When lungs of different animals are placed side by side the 

 various microscopic subdivisions in the more complex lungs 

 must be homologized with the macroscopic subdivisions of the 

 simpler lungs. Due to the many names which have been 

 already employed in the description of the various portions 

 of the lungs in different animals, the way in which I use 

 the term "air-sac" may be confusing to the reader. It seems 

 to me, however, that it would be more confusing if many new 

 terms were introduced. The plan has been to call the terminal 

 subdivision the air-cell ; the first cluster of these the air-sac ; 



1 I have already given a full history of the structure of the lung in the article 

 "Lung" in Buck's Reference Handbook of the Medical Sciences — SM^^lement 

 volume. Wm. Wood & Co., New York, 1893. 



