CHAP, xvi.] % PHYSIOLOGICAL OFFICE OF RESPIRATION. 241 



phers, who attributed intentions to nature, we should say that 

 this was the idea of respiration. 



The most easy conditions of the exchange of gases being best 

 realised amongst the higher mammifers, we can take as a type 

 respiration in man, the only one indeed which has yet been 

 thoroughly studied. Moreover it will be easy for us to indicate, 

 in the course of this exposition, the analogies or differences 

 observable in the remainder of the animal kingdom. 



The capillary vessel is preeminently the seat and agent of 

 material exchanges in the animal organism. "Wherever it may 

 be, it adapts itself readily to the osmotic phenomena. In the 

 web of the tissues, when the capillary partition separates liquids, 

 it is these liquids which mingle through the thin intermediate 

 membrane. In the lungs of man, the wall of the capillaries 

 separates the sanguineous liquid, holding gases in solution, from 

 the exterior atmosphere, and the exchange is made between the 

 gases. 



In the capillaries of the lungs (Fig. 34), as in those of the 

 tissues, the electro-motor currents play an important mechani- 

 cal part. But whilst in the tissues they are the agents of the 

 expulsion of the oxygen across the wall of the capillaries, in the 

 lungs they are precisely the reverse. In effect, in the lungs, the 

 oxygen is situated outside the circulation ; consequently the 

 direction of the electric current is inverse. The electro-motory 

 capillary- current tends therefore to cause the expulsion of the 

 carbonic acid. 1 



In spite of the action of breathing in the lungs of the verte- 

 brated animals, and especially of the mammifers, though there 

 are constantly and regularly an alternate entrance and exit of gas 

 in the pulmonary cavity, respiration, gaseous exchange through 

 the capillary wall, is nevertheless a continuous uninterrupted 

 phenomenon. In effect, at each expiration the lungs expulse 

 only a very small portion of their gaseous contents. According 



1 Becquerel, loc. cit. t and Des Forces Physico-chimiques, 8vo. Paris, 1875. 

 Onimus, Des Phenom&nes Electro-capillaires (Eevue Scientijique, 1870, n 42). 



B 



