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A MANUAL OF PHYSIOLOGY 



the elastic tension of the lungs. 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 intra- 

 thoracic pressure is not necessarily favourable to the circula- 

 tion, since the auricles are then unable to contract 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 cir- 

 culation both in inspiration and in expiration. But the increased 



work of the inspiratory 

 muscles may counter- 

 balance the advantage. 



Valsalvas experiment, 

 which is performed by 

 closing the mouth and 

 nostrils after a previous in- 



FIG. 89. PULSE-TRACING IN VALSALVA'S spiration, and then forcibly 

 EXPERIMENT (ROLLETT). trying to expire, is an imi- 



tation of breathing into 



compressed air. The intra-thoracic 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 (Fig. 89). 



Mullers experiment, which 

 should be bracketed with Val- 

 salva's, consists in making, after a 

 previous expiration, a strong in- 

 spiratory effort with mouth and 

 nostrils closed. Here the intra- 

 FIG. 90. PULSE-TRACING IN MULLER'S thoracic pressure is greatly dimin- 

 EXPERIMENT (ROLLETT). ished, more blood is drawn into 



the chest, and upon the whole 



effects opposite to those of Valsalva's experiment are produced 

 (Fig. 90). Neither experiment is quite free from danger. In both 

 the dicrotism of the pulse becomes more marked. 



When the whole body is subjected to the changed 

 pressure, as in a balloon or on a mountain, in a diving-bell 

 or a caisson used in building the piers of a bridge, the 



