SECRETION AND EXCRETION. 
125 
like the pancreas of Vertebrates in function, as its secre¬ 
tion digests starches and albuminoids. The liver of Ver¬ 
tebrates is both a secretory and an excretory organ. The 
bile performs an important, although ill-understood, func¬ 
tion in digestion, and also contains some waste products. 
The gland also serves to form sugar from part of the 
digested food, and may well be called a chemical work¬ 
shop for the body. In animals of slow respiration, as 
Crustaceans, Mollusks, Fishes, and Reptiles, fat accumu¬ 
lates in the liver. “Cod-liver oil” is an example. 
The great Excreting Organs are the lungs, the kid - 
neys , and the skin ; and the substances which they re¬ 
move from the system—carbonic acid, water, and urea— 
are the products of decomposition, or. organic matter on 
its way back to the mineral kingdom. 70 Different as these 
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 carbon dioxide and water, 
with a trace of urea; the kidneys expel water, urea, and 
a little carbon dioxide; whil§ the skin partakes of the nat¬ 
ure of both, for it is not only respiratory, especially among 
the lower animals, but it performs part of 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 the tis¬ 
sues and with some elements of the blood. Thus are pro¬ 
duced heat and work, whether muscular, nervous, secre¬ 
tory, etc. As a result of this oxidation, carbon dioxide, 
water, and urea or a similar substance, are poured into the 
blood. The carbon dioxide and part of the water are 
passed off from the respiratory organs. This process is 
