RESPIRATION. 231 



should either be wanting, or of the greatest delicacy ; and that 

 no lymph-corpuscles should be visible swimming in it apart 

 from the general current, but that they should be observed 

 mingled with the common stream (fig. 230 a, b, c).]* 



[ 3.Q2. The organs which serve in man and the various 

 classes of animals for respiration, and the mechanical part of 

 the function of these organs, have now been described. The 

 very essence of respiration, however, consists in this : that the 

 air of the atmosphere brought into contact with the blood 

 within the lungs effects certain changes in that fluid which are 

 indispensable to the maintenance of life. The air, it is true, 

 does not come into direct contact with the blood even in the 

 lungs, but is separated from it by the parietes of the pulmo- 

 nary cells and the walls of the blood-vessels. The air, how- 

 ever, readily penetrates these moist tissues, for it combines 

 with the watery fluid which permeates them, and so makes its 

 way even immediately to the blood. f As the lungs contain 

 air at all times, the influence which the elastic fluid exerts 

 upon the blood, and the changes w T hich the blood undergoes, 

 are not connected with the alternate assumption and rejection 

 of so much air. These are but means to an end : the proper 

 respiratory process, or that process for which inspiration and 

 expiration are instituted, goes on incessantly. Inspiration 

 and expiration are merely provisions for changing the air, 

 which must be renewed at intervals, longer or shorter, if the 

 object of respiration is to be attained. Before entering on 

 the peculiar chemical processes occurring in respiration, it is 

 proper to inquire into the changes which, 1st, the air, and 2nd, 

 the blood, experience in its course. 



[ 393. The earliest accurate researches into the nature of 

 respiration, were instituted with a view to determine the 

 changes which the air experienced in passing through the 

 lungs, and our information upon this part of the function 



* Professor Wagner's Physiology, pp. 358, et seq. 



f The penetration of the moist parietes of the air-cells and blood- 

 vessels is a general physical phenomenon, and independent of any peculiar 

 power or property inherent in the lungs ; any moist animal membrane 

 without or within the living body is gradually penetrated by the air of the 

 atmosphere and other gases. ( 413). The extensive subdivision which the 

 blood undergoes in the minute vessels of the lungs is obviously calculated 

 greatly to assist the operation of the air. 



