Mr. Le Gros Clark on the Mechanism of Respwation. 123 



may be accepted as an approximate measure of the elasticity of these 

 organs. 



(e) That the diaphragm was rendered flaccid by the admission of air 

 into the pleurae, independently of the collapse of the lungs. 



4. Thus there exists a constant antagonism between the atmospheric 

 pressure on the interior of the lungs and their elastic and contractile pro- 

 perties, tending to the expulsion of the air they contain; and this expul- 

 sive power is limited by the resisting tension of the diaphragm. These 

 several conditions are in no degree influenced by the direct admission of 

 air into the abdominal cavity ; for the trachea being treated as in the 

 former experiment before the abdomen was opened, and an aperture being 

 then made into the peritoneum, the abdominal walls rose as air rushed in, 

 but no air was expelled from the lungs. The expansion of the abdomen 

 under these circumstances is especially remarkable in oxen when turned 

 on their back, and is probably in great measure due to the position favour- 

 ing the expansion of the lower costal region. 



5. In the recognition of the above facts, I think that the extremely 

 tense condition of the diaphragm has not received the notice which its 

 practical importance deserves. Independently of being the agency by 

 which the supplemental air is retained in the lungs, the resistance thus 

 offered limits the encroachment of the abdominal viscera on the thoracic 

 cavity ; and by this same property, combined with the broad and insepa- 

 rable attachment of the central tendinous expansion to the fibrous peri- 

 cardium, the heart is preserved from being impeded in its movements in 

 the various stages of respiration. 



That the tension in question fulfils these functions is exemplified patho- 

 logically as well as physiologically. It would indeed be impossible, 

 without giving due weight to this property of the diaphragm, to account 

 for the trifling interference, comparatively speaking, that pregnancy, or 

 the presence of large ovarian tumours or ascitic accumulations, exercises 

 over the mechanism of respiration ; and it would appear still more sur- 

 prising that the sudden removal of such distension, and with it the pres- 

 sure on the diaphragm, is not attended by more serious consequences. Yet 

 the respiratory mechanism is scarcely influenced appreciably even by the 

 sudden collapse of an enormous ovarian cyst ; and this is accounted for by 

 the passive tension of the diaphragm, which alike resists encroachment 

 from below and refuses the solicitation to descend which the removal of 

 the pressure would seem to offer. 



6. In the action of the diaphragm this tension is an essential condition, 

 in order that there may be no waste of power, but that its earliest con- 

 traction may be accompanied, at once and simultaneously, by the entrance 

 of the tidal air into the lungs; and it would seem probable that, for ordi- 

 nary expiration, the suspension of its contraction and consequent return 

 to its normal state of arched tension is alone sufficient. 



7. But further considerations present themselves in reviewing this pro- 



L 2 



