STRUCTURE OF THE LUNGS. 155 



CHAPTER 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 continually 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 ab- 

 sorption. 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 degenerations 

 of tissue, or in part also from the elements of unassimilated 

 food. One of the most important and abundant of the impu- 

 rities is carbonic acid, the removal of which and the introduc- 

 tion of fresh quantities of oxygen, constitute the chief purpose 

 of respiration a process which, because of its intimate rela- 

 tion to the circulation, 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 upper- 

 most of the two cavities into which the body is divided by the 

 diaphragm (Fig. 31). 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 air-tubes may have 

 been cut across, and show, on their surface of the section, their 

 tubular structure. 



In fact, however, the lungs are hollow organs, and we may 

 consider them as really two bags containing air, each of which 

 communicates by a separate orifice with a common air-tube 

 (Fig. 31 ), through the upper portion of which, the larynx, they 

 freely communicate with the external atmosphere. The orifice 



