MECHANICAL CONDITIONS. 327 



the interchange of air is therefore considerably impeded from the 

 narrow calibre of the glottis being the only means by which air 

 can enter into and be expelled from this wide, conically-shaped 

 cavity. The high diffusibility of the gases certainly contributes in 

 some degree to counterbalance the effect of this narrow opening, 

 and induces no inconsiderable interchange of air, both within and 

 externally to this space ; but this motion would be quite insufficient 

 for the purposes of life (excepting in the case of the hybernating 

 animals), and hence special provisions exist for the expulsion of 

 the contained air by a partial diminution of this cavity and for 

 the re-admission of new air by its re-expansion. The mechanism 

 by which this object is effected depends partly upon the peculiar 

 structure of the thorax and the position of the muscles which 

 move it, and partly on the peculiar elasticity of the pulmonary 

 tissue. We should, however, form a very erroneous idea of the 

 motion induced by this mechanism, were we to conceive that it 

 was able to agitate the whole of the air contained within the cavity 

 of the chest. For even when the contraction is relatively con- 

 siderable, only a small fraction of the air is expelled, and an 

 equally small proportion admitted by its expansion ; hence it is 

 only in the wider air-canals that the air can be absolutely changed, 

 whilst in the narrower vessels there is only an undulating current 

 of the stagnant air-column, induced by the contractility of the 

 walls. The change therefore depends solely upon the different 

 degrees of diffusibility of the gases. However simple this latter 

 circumstance may appear, Vierordt has the merit of being the first 

 who experimentally illustrated these physical relations. 



A careful consideration of the above-mentioned mechanical rela- 

 tions shows how extensively this interchange of gases must be 

 modified by slight alterations of the external conditions. We 

 must not, therefore, believe that the difference in the characters of 

 the blood and of the inspired air merely influences the final results 

 of this interchange of gases, for it might rather be predicated as a 

 physical necessity, that if the blood were propelled more copiously 

 through the lungs, and the air was more frequently changed in 

 this conical space (the latter being more considerably dilated and 

 contracted), the results of the interchange would be different 

 from what they would be in the opposite case. A more careful 

 study of the mechanism of the respiration will not, however, always 

 enable us to refer the modifications which it presents to proximate 

 causes, based upon chemical and mechanical conditions ; for these 

 conditions are themselves often dependent upon so-called physio- 



