194 COMPARATIVE ZOOLOGY. 
organs appear, they are constructed upon the same prin- 
ciple: each consisting of a very thin sheet of tissue sepa- 
rating the blood to be purified from the atmosphere, and 
straining out, as it were, the noxious matters. All, more- 
over, excrete the same substances, but in very different 
proportions: the lungs exhale carbonic acid and water, 
with a trace of urea; the kidneys expel water, urea, and 
a little carbonic acid; while the skin partakes of the nat- 
ure of both, for it is not only respiratory, especially among 
the lower animals, but it performs the work of the kidneys 
when they are diseased. 
1. The lungs (and likewise gills) are mainly excretory 
organs. The oxygen they impart sweeps with the blood 
through every part of the body, and unites with, 2. e., 
burns up, the effete matters, which, set free by muscular 
and nervous exertion, would poison the system, if not 
removed. The carbonic-acid gas thus generated is car- 
ried by the veins to the lungs, and there exhaled in breath- 
ing. This process is more immedi- 
ately necessary to life than any oth- 
er: the arrest of respiration is fatal. 
2. While the lungs (and skin also, 
to a slight degree) are sources of 
gain as well as loss to the blood, the 
kidneys are purely excretory organs. 
Their sole function is to eliminate 
the solid products of decay which 
can not pass out by the lungs. In 
Mammals, they are discharged in 
solution; but from other animals 
Tre) 98. ection of Human 20), drink: little. the excretionaas 
ae a eapeemenet ay? more or less solid. In Insects, the 
cones; 7, the ureter, or out- kidneys are groups of tubes; in the 
yy ieee ce higher Mollusks, they are represent- 
ed hy spongy masses of follicles; in Vertebrates, they are 
