NATURE OF THE RESPIRATORY PROCESS. 325 



forming a scientific estimate of the quantitative metamorphosis of 

 matter, and of the entire animal economy. On this account we 

 consider it as the last link in the series in which we have treated 

 of the solid and fluid parts of the animal body and its various 

 functions. 



We should greatly err, were we to regard the lungs simply as 

 organs of excretion, for they differ from other excreting organs, 

 inasmuch as they not only excrete gaseous bodies, but also absorb 

 certain elastic fluid substances. An interchange of gases is there- 

 fore effected within the lungs. Although our notice of the respira- 

 tory process must necessarily be especially directed to the higher 

 pulmoniferous animals, we must bear in mind that that inter- 

 change of gases which we term respiration is by no means solely 

 limited to the organs known as lungs. As is well known, there 

 are many aquatic animals besides fishes which breathe through 

 gills, while insects are provided with a system of tubes (tracheae) 

 which take the place of lungs, and which, like blood-vessels, are 

 distributed through all the tissues on which vital activity depends 

 in these non-vascular animals a circumstance of great importance 

 in connection with the theory of the respiratory process. 



If the respiratory process depends essentially upon an inter- 

 change of certain substances within the lungs, an accurate 

 acquaintance with the substances whose constituent parts are thus 

 interchanged is indispensably necessary for the scientific com- 

 prehension of this subject. Thus, in considering pulmonary 

 respiration, we must be accurately acquainted, on the one hand, 

 with the constitution of the blood, and, on the other, with the 

 character of the atmosphere, before we can venture to form a 

 judgment regarding the interchange which takes place. We do 

 not, however, purpose entering more fully into the close con- 

 sideration of these two fundamental bases of the respiratory pro- 

 cess, as we have already, in the second volume, considered the 

 constitution of the blood and the amount of gases which it con- 

 tains ; and as we must presume that our readers are equally well 

 acquainted with the chemical constitution of the atmospheric air 

 and with the physical laws of the motion of elastic fluids. 



The causes which present themselves to our notice as essential 

 agents in effecting that interchange of gases which takes place in 

 the lungs of air-breathing animals may be of three different kinds, 

 anatomico-mechanical, physico-chemical, and purely physiological, 

 without, however, being fundamentally and completely distinct; 

 for each cause must, from the nature of the case, merge into the 



