326 RESPIRATION. 



others. We need scarcely observe that the mechanical momenta 

 depend upon the manner and the degree in which the hodies acted 

 upon, namely, the blood and the atmospheric air, are brought into 

 contact with one another, and by which this interchange is ren- 

 dered possible and promoted. We abstain from giving a systematic 

 exposition of the anatomical relations and the entire mechanism of 

 the respiration, since, in the first place, we must presume that our 

 readers have some knowledge of anatomy as well as of pure 

 chemistry, and, in the second, that the following remarks will 

 suffice to afford some idea of these modifications in the quantitative 

 results of the respiratory process, which depend upon differences 

 in the mechanical relations. 



In the first place, we ought to observe that no direct com- 

 munication exists between the fluids which undergo this mutual 

 interchange, for the elastic and fluid atmosphere and the liquid 

 blood, are separated by an extremely delicate moistened membrane. 

 Although the interchange of the gases may be somewhat retarded 

 by these membranes, nature has compensated for these impedi- 

 ments by giving an extraordinary degree of expansion to the 

 surfaces of contact. The extremely delicate distribution of the 

 blood- and air-vessels affords an immense extent of superficies in a 

 small space, and enables the processes to be widely diffused. The 

 fluids, however, do not stagnate at those surfaces (the membranes) 

 which establish communication between them, but both are main- 

 tained by very different physical means in continuous motion and 

 in constant interchange ; and hence we have an additional condition 

 which essentially facilitates this process. The heart, which, as 

 is well known, sends forth the vessels of the lesser circulation into 

 the lungs, constantly propels new blood through them, and is un- 

 doubtedly the most essential agent in circulatingthe blood through 

 these organs ; for even though the mechanism of respiration 

 may exert some action on the movement of the blood in the 

 lungs, pathologists have gone too far in maintaining, as some 

 have done, that respiration is the sole cause of the motion of the 

 blood. With regard to the motion of the air, on the other hand, 

 it must be observed that the great increase of surface obtained in 

 a circumscribed space (by the minute division of the air-vessels) 

 renders its interchange with the gases of the blood less easy ; we 

 may compare the space filled with air within the thoracic cavity 

 to a cone whose base is extremely large in proportion to its 

 height ; the base being constituted by the sum of the surfaces of 

 the pulmonary vesicles, whilst we place the apex in the glottis ; 



