296 REBP1RA TlOtf 



Breathing compressed air should, therefore, under the conditions 

 described, be upon the whole unfavourable to the venous return to the 

 heart and to the filling of the arteries, and the arterial pressure should 

 fall ; while breathing rarefied air should have the opposite effect. But 

 a very great diminution of the intrathoracic pressure is not necessarily 

 favourable to the circulation, since the auricles are then unable to con- 

 tract perfectly. 



Certain chest diseases have been treated by the use of apparatus by 

 which the patient is made to breathe either compressed or rarefied air ; 

 or to inspire air at one pressure and to expire into air at another pressure. 

 And it has, upon the whole, been found, in agreement with theory, 

 that condensed air cannot help the circulation however it is applied, but 

 always hinders it; while rarefied air aids the circulation both in inspira- 

 tion and in expiration. But 

 the increased work of the in- 

 spiratory muscles may coun- 

 terbalance the advantage. 



Valsalva's experiment, 

 which is performed by closing 

 the mouth and nostrils after 

 a previous inspiration, and 

 Fig. 132. Pulse Tracing in Valsalva's Experi- then forcibly trying to expire , 

 ment (Rollett). is an imitation of breathing 



into compressed air. The 



intrathoracic pressure is raised, it may be, to considerably more than 

 that of the atmosphere; the venous return to the heart is impeded, 

 and may be stopped ; and the pulse curve is altered in such a way as 

 to indicate first an increase and then a decrease of the arterial blood- 

 pressure succeeded by a second rise (Fig. 132). 



Muller's experiment, which should be bracketed with Valsalva's, 

 consists in making, after a previous expiration, a strong inspiratory 

 effort with mouth and nostrils closed. Here the intrathoracic pressure 

 is greatly diminished, more blood is drawn into the chest, and upon the 

 whole effects opposite to those of Valsalva's experiment are produced 

 (Fig. 133). Neither experiment is 

 quite free from danger. In both 

 the dicrotism of the pulse becomes 

 more marked. 



When the whole body is sub- 

 jected to the changed pressure, 

 J . , ,, Fig. 133. Pulse Tracing in Muller s 



as in a balloon or on a mountain, Experiment (Rollett). 



in a diving-bell or a caisson used 



in building the piers of a bridge, the conditions are very different. 

 For the blood-pressure, the intrathoracic pressure, and the intra- 

 alveolar pressure, all fall together when the pressure of the atmo- 

 sphere is diminished, and all rise together when it is increased. It 

 is possible not only to live, but to do hard manual labour, at very 

 different atmospheric pressures. 



As regards the chemical effects of condensed and rarefied air, 

 Loewy found that the quantity of oxygen absorbed by a man breath- 

 ing air in the pneumatic cabinet remained constant at all pressures 

 between about two atmospheres and half an atmosphere. At 440 mm. 

 of mercury dyspnoea became evident ; but if the person was now made 

 to work, the dyspnoea passed away, and did not again manifest itself 



