197 

 CHAPTEE VII. 



RESPIRATION. 



As the blood circulates through, the various parts of the 

 body, and fulfils its office by nourishing the several 

 tissues, by supplying to secreting organs the materials 

 necessary for their secretions, and by the performance of 

 other duties with which it is charged, it is deprived of 

 part of its nutritive constituents, and receives impurities 

 which need removal from the body. It is, therefore, 

 necessary that fresh supplies of nutriment should be con- 

 tinually added to the blood, and that provision should 

 be made for the removal of the impurities. The first of 

 these objects is accomplished by the processes of digestion 

 and absorption. The second is principally effected by the 

 agency of the various excretory organs, through which are 

 removed the several impurities with which the blood is 

 charged, whether these impurities are derived altogether 

 from the degeneration of tissues, or in part also from the 

 elements of unassimilated food. One of the most important 

 and abundant of the impurities is carbonic acid, the re- 

 moval of which and the introduction of fresh quantities of 

 oxygen, constitute the chief purpose of respiration a 

 process which, because of its intimate relation to the cir- 

 culation, may be considered here, rather than with the 

 other excretory functions. 



Position and Structure of the Lungs. 



The lungs occupy the greater portion of the chest, or 

 uppermost of the two cavities into which the body is 

 divided by the diaphragm (fig. 51). They are of a spongy 

 elastic texture, and on section appear to the naked eye as 

 if they were in great part solid organs, except here and 

 there, at certain points, where branches of the bronchi or 



