433 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



ber of the pores has been variously estimated at from 

 five hundred to twenty-eight hundred to the square inch, 

 and from one and a half million to seven millions on 

 the whole surface of the body. The aggregate length 

 of the tubes has been estimated at twenty-eight miles.* 

 Like all infolded surfaces, the tubes are lined throughout 

 with living epithelial cells, which preside directly over 

 the excretion. Blood vessels furnishing the materials 

 are distributed over the exterior surface of the tubes. 



It is quite possible that the number and aggregate 

 length of the uriniferous tubes may be as great as that 

 of the sudoriferous, but in the one case these are com- 

 pacted into an organ (the kidney), and the excretion 

 gathered into a reservoir (the bladder), while in the 

 other they are spread over the wide surface of the skin. 

 The reason is obvious. The purpose of cooling the 

 blood by evaporation could only be subserved by a large 

 surface of direct exposure. 



Lungs, Kidneys, and Skin compared. — All three 

 of these eliminate C0 2 , H g O, and urea. But the distinc- 

 tive duty of the lungs is the elimination of C0 2 , that of 

 the kidneys urea, and that of the skin water in such wise 

 as to cool the blood. In each case the elimination of 

 the other two products is subsidiary and, as it were, ac- 

 cidental. Nevertheless, we ought not to be surprised to 

 find a mingling of these functions more and more as we 

 go down the scale. 



COMPARATIVE MORPHOLOGY AND PHYSIOLOGY OF 

 THE SKIN. 



Genera Remarks. — i. If the skin be dry, harsh, 

 and without glands, it is a sign either that cooling of 

 the blood is not necessary, as in cold-blooded animals, 



* Nature, 1, 257, 1894. 



