INTRODUCTORY OBSERVATIONS. 83 



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 branchiae 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 eifected 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' 



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