20 Mr. Carlisle's Account of a peculiar Arrangement 
arteries of the alimentary canal in other animals ; and, in many 
of them, this extension of undivided vessels is to be found. 
See Fig. 2. 
Many of the amphibious class of animals are slow in their 
motions, such as the tortoises, lizards, and toads ; but, whether 
they can also continue their muscular actions longer than in 
ordinary cases, I do not know. The blood circulates more 
slowly in the amphibia ; and their respiration is not so important 
to the vital functions. It cannot however be omitted, that many 
of the serpents, and some lizards, are very agile. 
During the state of contraction, as has been often noticed, 
the muscles of animals with red blood become of a paler colour, 
and recover their former redness on the subsequent relaxation ; 
it may therefore be affirmed, that muscular fibres are not dis- 
tended with blood when in the state of contraction, but that the 
replenishing with blood is to supply some fresh material, which is 
employed in muscular action. Whatever substance this maybe, 
it is less required for the irritability of the muscles in some ani- 
mals than in others. Temperature, and the organs of respiration, 
seem to be intimately connected with these differences ; but all 
illustrations of such points would extend beyond the limits of 
the present inquiry. 
It has been shewn, that slowness of muscular action, and 
extraordinary duration of the contractions, are frequently united ; 
and that such unusual phenomena in muscles, are accompanied 
with a peculiar distribution of the arteries which supply them : 
but, whether the slowness or the duration be the principal end, 
or whether the equable supply of blood by a set of appropriate 
arteries, be the only adaptation convenient for the peculiar offices 
of such muscles, are subjects not easily determined. 
