392 RESPIRATION. 



large portion of the so-called free oxygen and carbonic acid gas in 

 the blood is in a state of unstable chemical combination, and 

 hence Dalton's law can, strictly speaking, only apply to the fraction 

 of carbonic acid and oxygen contained in the blood which is only 

 mechanically absorbed, or, to express the same thing in different 

 words, is merely taken up by the water in the blood. The law of 

 absorption is not, however, of less importance to the theory of 

 respiration ; and we may perhaps be justified in assuming that all 

 the oxygen is mechanically absorbed (in accordance with the 

 above-mentioned law) before it enters into this unstable chemical 

 combination, and that the carbonic acid, before it is separated 

 from the blood, is mechanically dissolved from its chemical com- 

 bination, when the diminished external pressure favours, or rather 

 controls, its elimination. It cannot, however, as yet be strictly 

 proved that the membranes which separate the blood and the air 

 within the lungs may not manifest different degrees of permeability 

 towards the gases or the fluids which saturate them, and may not, 

 therefore, exert some influence on the interchange of gases proceed- 

 ing in accordance with the law of absorption ; for although these 

 membranes may be extremely thin, they yet consist of at least three 

 delicate layers of tissue, namely, the pavement epithelium and the 

 membrane propria of the pulmonary vesicles, and the walls of the 

 capillaries. The great difference of permeability shown by animal 

 membranes towards fluids, having even the same character, makes 

 it not unreasonable to conclude that, if we could once succeed in 

 establishing a general formula for the expression of the inter- 

 change of gases in the lungs, this function would constitute a 

 part of it. 



The effects of the respiration on the entire metamorphosis of 

 animal matter, and on the individual functions of the latter will be 

 systematically considered in the following section. It is customary 

 to associate the theory of animal heat with that of the respiration, 

 for, since the time of Priestley and Lavoisier, flame has not been 

 regarded merely as a poetical symbol of life, but life and com- 

 bustion have been regarded as two perfectly similar processes. 

 Lavoisier's theory of animal heat has experienced various modifi- 

 cations in the course of time and from the pressure of advancing 

 science, as has also the theory of combustion, although both are 

 true in their fundamental principles. We will not here enter 

 more fully into the theory of animal heat, since it still rests on a 

 very uncertain foundation, and since further an accumulation of 

 the various facts and arguments bearing upon the subject would 



