xi I. J 1 'HE EXCKJ:. '1 'OR Y ORGA.\ 'S. 46 1 



LESSON XII. 



THE EXCRETORY ORGANS. 



i. IN the First Lesson of " Elementary Physiology," 23, 

 while explaining the nature of the excretory process and its 

 most general conditions, it was stated that the EXCRETORY 

 ORGANS are three in number, namely : (i), the skin ; (2), the 

 lungs ; and (3), the kidneys. The description of the skin and 

 its various appendages has formed the subject of our Seventh 

 Lesson. Nevertheless, the consideration of its glandular 

 structure was purposely deferred to the Twelfth Lesson, and 

 may be here considered, with the lungs and kidneys together 

 with adjoined and adjacent structures not yet treated of. 



The lungs, however, are importers of oxygen (for the 

 aeration of the blood) as well as excretory organs ; it is not, 

 then, surprising that the external skin may assume that func- 

 tion also, as in fact we found (when considering the circulating 

 system) to be the case in Batrachians. 



The function of importing oxygen into the blood is per- 

 formed in many animals by a set of organs of which no repre- 

 sentative exists in man, and which, therefore, are not referred 

 to in the Lessons of " Elementary Physiology," though they 

 must occupy no inconsiderable portion of our concluding 

 Lesson of " Elementary Anatomy." The organs in question 

 (mentioned in our First and Tenth Lessons) are the gills, or 

 branchia. These are delicate processes of skin, richly supplied 

 with blood and capable of absorbing oxygen, not by the 

 decomposition of the water in which they float, but by the 

 absorption of oxygen from the particles of air which are mixed 

 up with and dissolved in that water. Such structures, as might 

 be expected, are only found in animals which live in or 

 frequent water, whether fresh or salt, though not all animals 

 which frequent water have gills. 



Thus, in being destitute of branchiae, man agrees not only 

 with the whole of his class (including, of course, the Whales 



