448 Historical 



considerably reduced the maximum capacity which the lungs can 

 attain under ordinary pressure, and consequently the usual expansion 

 of the pectoral cavity; then is it not manifest that in increasing this 

 pressure and thus raising to greater power the energy which struggles 

 against the reaction of the lungs, we should extend the upper limit of 

 its own development and consequently that of the expansion of the 

 thoracic framework under the effort of the inspirator muscles, an effort 

 which promptly loses its power when the tendency towards a vacuum, 

 which takes place between the two pleura during the inspiration, 

 passes a certain limit? 



2. Since the increase of the atmospheric pressure has the effect 

 of compressing the abdomen, of increasing the elasticity of the intes- 

 tinal gases and consequently their reaction against the effort of the 

 diaphragm, this muscle meets a more stable point of support and 

 changes the usual mode of respiration, forcing the ribs and the 

 sternum to take a greater part in the mechanism of this function. In 

 fact, the expansion of the thoracic cavity vertically is thus diminished; 

 but this reduction is more than compensated for by the expansion of 

 the chest, following its antero-posterior and lateral diameters, and, far 

 from being lessened, the volume of air admitted by each inspiration 

 is increased. In fact, in the mode of respiration which takes place 

 principally by the descent of the diaphragm, the capacity of the chest 

 increases only according to the simple proportion of the successive 

 vertical diameters, measured laterally, for the middle part of the dia- 

 phragm remains almost stationary; whereas in the costo-sternal respi- 

 ration, the enlargement of this cavity takes place in the compound 

 proportion of the product of the original horizontal diameters to the 

 product of the same diameters expanded. (P. 11-12.) 



B. Hematosis is expedited: 



Is it, as is generally thought, because the compressed air contains 

 in a given volume a greater absolute quantity of oxygen that it expe- 

 dites and improves the oxygenation of blood? (P. 21.) 



Pravaz then compares the recent experiments of MM. Regnault 

 and Reiset with the former statements of Allen and Peppys, and 

 he adds: 



If, between these contradictory statements, one leaned towards the 

 former, as guaranteed by experimenters with the reputation of greater 

 exactness, one would not be puzzled for an explanation of how com- 

 pressed air can give other results than pure oxygen, or oxygen merely 

 offered in greater quantity for pulmonary absorption. 



In fact, Lavoisier and MM. Regnault and Reiset collected their 

 observations at ordinary pressure; now we know, on the authority of 

 M. Biot, that the quantity in weight of gases dissolved in a liquid 

 increases proportionately to the pressure which these gases support. 



There is therefore in the action of compressed air on the organism 

 an element other than the multiplication of the molecules of oxygen 

 in a given volume; this element is a mechanical force greater than 

 that which acts upon the gases under experimentation at the ordinary 



