RESPIRATION 197 



executed by beheading, was 457 grm. (Gluge). The average weight of 

 the lungs in a great number of persons v/ho had died a natural death 

 was 1024 grm. (Juncker). The weight of the pulmonary tissue alone 

 in the first set of cases must be less than 457 grm., for the lungs of 

 a person who has bled to death are never bloodless. In a dog killed 

 by bleeding from the carotid, one-quarter of the weight of the lungs 

 consisted of blood. Assuming the same proportion for the de- 

 capitated individuals, we get 343 grm. as the net weight of the blood- 

 free lungs. Deducting this from 1024 grm., we arrive at 68 1 grm. 

 as the average quantity of blood in the lungs. Adding to this the 

 quantity in the right side of the heart (p. 127), we get, in round 

 numbers, 750 grm. as the amount in the lesser circulation. It is 

 true that in the living body the conditions are not the same as after 

 death ; but it is probable that in a large number of cases taken at 

 random the differences would be approximately equalized. 



It has been further calculated but here the data are less certain 

 that the total area of the alveolar surface of the lungs of a man is 

 about 100 square metres (sixty times greater than the area of skin), 

 of which, perhaps, 75 square metres are occupied by capillaries. 

 The average thickness of this immense sheet of blood has been 

 reckoned to be equal to the diameter of a red blood-corpuscle, or, 

 say, 8/A. This would give 600 c.c. (630 grm.) as the quantity of 

 blood in the lungs, which is probably somewhat too low an estimate. 



If we take the pulmonary circulation-time as 13 seconds (p. 124), 



and the quantity of blood in the lungs as 800 grm., then 



-221 kilos of blood will pass through the lungs in an hour, or 

 5,304 kilos (say, 5,000 litres) in twenty-four hours. This would fill 

 a cubical tank in which the man could just stand upright with the 

 lid closed. 



Mechanical Phenomena of Respiration. 



The lungs are enclosed in an air-tight box, the thorax; 

 or it may be said with equal truth that they form part of 

 the wall of the thoracic cavity, and the part which has 

 by far the greatest capacity of adjustment. The alveolar 

 surface of the lungs is in contact with the air. The pleura, 

 which covers their internal surface, is reflected over the 

 chest-walls and diaphragm, so as to form two lateral sacs, 

 the pleural cavities. In health these are almost obliterated, 

 and the visceral and parietal pleurae, separated and 

 lubricated by a few drops of lymph, glide on each other 

 with every movement of respiration. But in disease the 

 pleural cavities may be filled and their walls widely separated 

 by exudation as in pleurisy, or by blood as in rupture of an 



