148 CARBON DIOXIDE IN EXPIRED ALVEOLAR AIR, 



The uniformity in the figures yielded in tiie last two analyses 

 was considered as a proof that the air w as derived from the 

 alveoli unmixed with that of the "dead space" of the trachea and 

 respiratory passages lying between the atmosphere around the 

 body, and the alveoli of the lungs. 



Haldane further presented a number of analyses (about 54 in 

 all) made on two subjects with the object of measuring the mean 

 composition of the alveolar air. The figures were obtained with 

 the subject at rest in a sitting position, w^iile breathing fresh 

 air at normal atmospheric pressure. The mean concentrations of 

 carbon dioxide were 5'62%, and 6' 28% for the two persons, the 

 maximal and mimimal variations being 5-40% and 5-87%, and 

 5 -985% and 6*845% respectively. Haldane commented on the 

 constancy of these figures, which he considered clearly suggested 

 that the ventilation of the lungs during rest was regulated so as 

 to maintain the percentage of carbon dioxide in the alveolar air 

 at an almost fixed level. 



In 1893, W. S. Miller *6) made a study of the structure of the 

 human lung, after he had studied the morphology in Necturus, 

 the frog, the snake, the crocodile, the turtle, Heloderma (lizard), 

 the fowl, cat, dog, rabbit and sheep. He employed dried speci- 

 mens of the lungs, corrosion-preparations in wax or Wood's 

 metal, and reconstructions from sections. The paper, which 

 remained little recognised for many years, now forms the basis 

 of our conception of pulmonary structure. The terminal bronchi- 

 ole opens out into a club-shaped expansion, from which five or 

 six openings, or "vestibula", lead to secondary expansions known 

 as "atria", which communicate with central cavities or "air-sacs" 

 set about with the small, irregular cubicles or "air-cells". The 

 "air-cells" correspond to the alveoli of physiological writers. 

 These structures are found not only in the walls of the air-sacs, 

 but also in those of the atria. Alveolar air, as understood by 

 physiologists, represents that part of the gaseous contents of the 

 lungs filling the "air-cells" belonging to the "air-sacs". 



The composition of the alveolar air has been made the subject 

 of repeated investigations by Krogh and Lindhard. Lindhard(7) 



