Summary and Discussion 497 



Variations in the respiratory amplitude and rhythm have also 

 been explained by the mechanical action of compressed air. 



Some, like Pravaz, have thought that compressed air promotes 

 pulmonary expansion, by opposing more energetically the elastic 

 reaction of the tissues. That is the converse of the theory sug- 

 gested in regard to decompression, the inexactness, or at least the 

 great exaggeration, of which we have shown. 



Others, with much better reason, have cited the effect of intes- 

 tinal gases. In fact, they form the only part of the organism upon 

 which the pressure of the air can act directly. Even though their 

 volume cannot increase in the phase of rarefaction, as we have 

 seen, because of the two orifices which allow an excess to escape 

 so easily, it can and evidently must decrease following the Law 

 of Mariotte, and indefinitely, as the outer air is more compressed. 

 And this really does take place; caisson workmen, whom I have 

 questioned, have told me that they were compelled, when once in 

 the caissons, to pull up the buckle of their pantaloons because of 

 the retraction of the belly. 



Although this fact has been established, I cannot accept the 

 conclusion which Pravaz draws from it (page 448), that the in- 

 creased elasticity of these gases hampers the action of the dia- 

 phragm, and decreases the vertical expansion of the thorax, but 

 increases the expansion of the chest in the other two directions. 

 Besides the fact that this hypothesis hardly seems tenable, the 

 measurements obtained directly by Vivenot by means of percus- 

 sion and auscultation show that in compressed air the lungs drop 

 lower than in the normal state. 



To disclose the mechanism of the increase of the thoracic cavity, 

 Panum performed an experiment which we reported above (page 

 480). It has only one defect, namely, that it was useless to make 

 it; certainly, if we are given a tube closed at its ends by two mem- 

 branes of unequal thickness, filled with water and containing be- 

 sides a bladder full of air, if this apparatus is put under pressure, 

 we shall see the bladder decrease in volume and the two mem- 

 branes pushed into the tube in inverse proportion to their thick- 

 ness. Very evidently something similar must take place in the 

 abdomen, between the gases of the intestines on the one hand and 

 the diaphragm and ventral wall on the other. The whole interest 

 of the question lies in knowing in what proportions these last- 

 mentioned organs tend to invade, pushing from without inward, 

 the space which was occupied by the intestinal gases, now dimin- 

 ished in volume. But Panum's experiment says nothing about that. 



