RESPIRA TION 



273 



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 inspiration and in expiration. But the increased work of 

 the inspiratory muscles may counterbalance the advantage. 



Valsalva's experiment, which is performed by closing the mouth 

 and nostrils after a pre- 

 vious inspiration, and 

 then forcibly trying to 

 expire, is an imitation 

 of breathing into com- 

 pressed air. The intra- 

 thoracic pressure is raised, 

 it may be, to considerably 



more than that of the FIG. 117. PULSE TRACING IN VALSALVA'S 

 atmosphere ; the venous EXPERIMENT (ROLLETT). 



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 (Fig. 117). 



Midler'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. 1 1 8). Neither 

 experiment is quite free from 

 clanger. In both the dicrotism 

 of the pulse becomes more 

 marked. 



When the whole body is 

 subjected to the changed pres- 

 sure, as in a balloon or on a 

 mountain, 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 atmosphere 

 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 breathing 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 till the pressure was 

 reduced to 410 mm. There are towns on the high tablelands 



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