326 PULMONARY MUCOUS TISSUE. 



of the mesenteric Veins, which were dark colored at the be- 

 ginning of the operation, are soon observed to become bright 

 red, like arterial blood ; this is because oxygenation has been 

 effected simply by the exposure to air, both of the surface of 

 the mesentery, and of the intestine during the experiment ; 

 and the frog, thus prepared, breathes (in the pulmonary or 

 respiratory sense of the word) through the lungs, the skin, 

 and the mesentery. In speaking of the epithelium of the 

 lungs, we have already mentioned that oxygenation goes 

 on in the intestinal mucous of the cobitis fossilis (water 

 loach). Finally, the skin of the superior animals, and even 

 of man, appears to have some share in the exchanges effected 

 by respiration between the blood and the outer air, especially 

 in respect to exhalation ; we shall return to this subject when 

 studying the functions of the cutaneous surface. 



These exchanges, however, for the most part, are made 

 on one particular surface, which, in the case of those animals 

 which live in the air, is represented by the lungs. 1 The 

 lungs are the organ of respiration, insomuch as they are the 

 place in which exchange goes on between the blood and 

 the outer air: respiration has been hitherto studied from this 

 point of view, but our present knowledge of the subject 

 allows us to regard the pulmonary function, not as the only 

 seat of respiration, but as representing a link, and as one of 

 the least important, in the long chain of processes which, 

 beginning in the very depth of the histological elements, 

 terminate in those surfaces which come in contact with the 

 external medium. 



The function of the pulmonary surface can thus be fully 

 understood only in the light of the recent acquisitions of 

 physiology; and the history of respiration offers a most 

 singular collection of hypotheses formed on this subject by 

 physiologists and physicians: some maintaining that the 

 pulmonary respiration has only a mechanical office, by which 



1 These exchanges take place in the epithelium of the bronchi, 

 as well as in that of the alveoli. The columnar epithelium of tho 

 bronchial mucous readily allows the production of hematosis (o/ 

 pulmonary gaseous exchanges). In order to prove this, we need 

 only remember the anatomical fact that the bronchial arteries have 

 no corresponding veins, and that their blood, having nourished the 

 bronchi, becomes oxidized by contact with the air, and, conse- 

 quently, flows immediately into the pulmonary veins, which latter 

 bring it back to the heart with the general mass of the blood that 

 has become arterial blood. 



