RESPIRATION 35 



two to five distributing chambers which he named "atria," and 

 which are also lined with alveoli. From each atrium a number of 

 openings lead onwards into what he calls "air- sacs," which are 

 main cavities of which the walls are also constituted of alveoli or 

 air cells. By far the greater part of the alveoli belong to the air- 

 sac system, but a certain number belong to the respiratory bron- 

 chioles, alveolar ducts and atria; and the latter act partly as air 

 passages to the air sacs, and partly perform the same respiratory 

 functions as the air sacs themselves. 



With this anatomical arrangement the whole of an air-sac sys- 

 tem is about equally well ventilated with fresh air, the only alveoli 

 which receive an undue supply of fresh air being those of the 

 respiratory bronchioles, alveolar ducts and atria. We can thus 

 understand why it is that the deeper parts of a very deep breath 

 have exactly the same composition as the middle parts. Evidently 

 however what Priestley and I called "alveolar air" is air-sac air. 



The fact that the atria, etc., have partly a respiratory function, 

 and partly act as air passages to the air-sac system, enables us 

 also to clear up some otherwise unintelligible facts with regard 

 to the "dead space" in breathing. The dead space was first esti- 

 mated roughly by Loewy from the volume of a cast of the 

 respiratory passages, taken in a human lung after death. As this 

 method seemed uncertain, Priestley and I made determinations by 

 comparing the composition of a whole breath of expired air with 

 the composition of what we took to be the whole alveolar air. We 

 calculated the expired air as a mixture of this alveolar air with 

 fresh air occupying the dead space. In this way we found that 

 during rest the volume of the "effective dead space" is about 

 30 per cent of the volume of the average tidal air. For greater 

 certainty Douglas and I collected the whole of the expired air 

 over a certain period, and made the same calculation from the 

 average volume and composition of each breath, compared with 

 the composition of the alveolar air. 19 We then found that the 

 "effective dead space" is far greater during the hyperpnoea of 

 hard muscular work than during rest. As we were then still un- 

 aware of Miller's work we interpreted our observations as indicat- 

 ing that the bronchi or other respiratory passages become wider 

 during hyperpnoea, so as to enable air to enter the lungs more 

 easily. Any one who examines a section of lung must be struck at 

 once by the fact that the mucous membrane of the bronchi is 

 usually in folds, indicating that if the muscular coat relaxed the 



19 Douglas and Haldane, Journ. of Physwl., XLV, p. 235, 1912. 



