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INTRODUCTORY OBSERVATIONS. 85 
conclusive. Six Frogs were taken for one of the experi- 
ments, and a ligature tied with the most rigid compression 
round the neck, so as to exclude any possible passage of air. 
One of these lived twenty days in a damp atmosphere, 
whilst those which were placed in five ounces and a half 
of water, died as soon as all the air contained in that 
water was respired, which was in from one to three days. 
Another experiment of corresponding tenour was per- 
formed by the total excision of the lungs; and of three 
Frogs thus treated, two died on the thirty-third, and one 
on the fortieth day. 
The results of other experiments proved that pulmonary 
respiration alone is not sufficient to support life without the 
aid of that of the cutaneous surface. 
It is very clear that this important function cannot be 
carried on unless the surface be constantly kept in a moist 
state. The branchiz of fishes and of crustacea, and the 
lungs of all pulmoniferous animals, equally require that the 
respiratory surface in every modification should be humid; 
for as soon as it becomes dry, its function ceases, and the 
animal speedily dies. But as the Frog, for instance, is 
frequently exposed to a dry atmosphere, it is essential that 
there should be some provision made for a constant supply 
of moisture to the skin, which has just been shewn to be a 
respiratory surface. This is effected precisely as in other 
surfaces which perform this function,—namely, by a secre- 
tion of fluid from the surface itself. The extent of the skin 
is, however, so great that the whole internal moisture of 
the animal would speedily be exhausted, unless a reservoir 
were provided for an extraordinary demand; and I now 
proceed to shew what this reservoir is, and by what means 
it is replenished. Every one knows that when a Frog is 
hastily seized, or even quickly pursued, it voids a consi- 
G 2 
