INTRODUCTORY OBSERVATIONS. 81 
habits of these species. Thus the cuticle of the Frog and 
often of the Water Newt comes off in shreds in the water ; 
whilst that of the Toad is removed at once by the exertions 
of the animal, comes away whole, and is swallowed at one 
gulp. Of this very curious fact the details will be found 
in the description of the common Toad. 
But the most interesting circumstance connected with 
the functions of the integuments of these animals, or in- 
deed with any part of their economy, is their cutaneous 
respiration ; or, in other words, the power which the sur- 
face of the skin possesses of effecting those changes in the 
blood which are usually performed by the lungs or branchiz. 
It was ascertained by Spallanzani, that the skin of the 
Frog gave out carbonic acid when in contact with the at- 
mosphere; but the subject had never undergone that 
strict and philosophical investigation by which all its impor- 
tant bearings should be ascertained, until Dr. William 
Edwards of Paris entered upon the inquiry, the results 
of which he published in his admirable work ‘On the 
Influence of Physical Agents on Life ;” than which there 
does not exist in the whole range of physiological literature 
a more perfect example of the manner in which a physio- 
logical inquiry ought to be imagined, planned, and con- 
ducted, and its results reasoned upon, and described. As 
I have endeavoured, in the article already quoted, to give 
a short sketch of the principal results of Dr. Edwards’s 
experiments, I shall now merely offer a somewhat modi- 
fied quotation from that article, rather than enter at length 
into the detail, which, however interesting the subject may 
be to the physiologist, would be out of place in the present 
work, and would be unnecessary to those who alone could 
appreciate it: for where is the physiologist who has not 
read and admired the original work itself? 
