224 RESPIRATION 



If we take the pulmonary circulation-time as 13 seconds (p. 137), 

 and the quantity of blood in the lungs as 700 grm., then Z 



= 194 kilos of blood will pass through the lungs in an hour, or 4,650 

 kilos (say, 4,400 litres) in twenty-four hours. This would fill a cubical 

 tank in which the man could almost stand upright with the lid closed. 



SECTION II. MECHANICAL PHENOMENA OF EXTERNAL 

 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 rilled and 

 their walls widely separated by exudation, as in pleurisy, or by 

 ,blood, as in rupture of an aneurism, or by air in the condition 

 known as pneumo-thorax. Between the two pleural sacs lies a mesial 

 space, the mediastinum, commonly divided into an anterior medias- 

 tinum in front of the heart, and a posterior mediastinum behind it. 

 The pleural and pericardial sacs and the mediastinum constitute 

 together the thoracic cavity. The external surface of the chest- 

 wall and the alveolar surface of the lungs are subjected to the 

 pressure of the atmosphere, to which the pressure in the thoracic 

 cavity (intra-thoracic pressure) would be exactly equal if its bound- 

 aries were perfectly yielding. But in reality the intra-thoracic 

 pressure is always normally something less than this. For even 

 the lungs, the least rigid part of the boundary, oppose a certain 

 resistance to distension, and so hold off, as it were, from the thoracic 

 cavity a portion of the alveolar pressure ; and in any given position 

 of the chest the intra-thoracic pressure is equal to the atmospheric 

 pressure minus this elastic tension of the lungs. 



The object of the respiratory movements is the renewal of the air 

 in contact with the alveolar membrane in other words, the ventila- 

 tion of the lungs. Two main methods are followed by sanitary 

 engineers in the ventilation of buildings: they force air in, or they 

 draw it in. In both cases the movement of the air depends on the 

 establishment of a slope of pressure from the inlet to the interior. 

 In the first method, this is done by increasing the pressure at the 

 inlet ; in the second, by diminishing the pressure at the outlet. In 

 certain animals Nature, in solving its problem of ventilation, has 

 made use of the first principle. Thus, the frog forces air into its 





