on muscular Motion. 
15 
It appears that all the classes of animals are endowed with 
some power of producing thermometrical heat, since it has 
been so established in the amphibia, pisces, vermes, andinsecta, 
by Mr. John Hunter ; a fact which has been verified to my 
own experience ; the term “ cold-blooded” is therefore only 
relative. The ratio of this power is not, however, in these 
examples, sufficient to preserve their equable temperature in 
cold climates, so that they yield to the changes of the at- 
mosphere, or the medium in which they reside, and most of 
them become torpid, approaching to the degree of freezing 
water. Even the mammalia, and aves, possess only a power 
of resisting certain limited degrees of cold ; and their surfaces, 
as well as their limbs, being distant from the heart, and prin- 
cipal blood-vessels, the muscular parts so situated are subject 
to considerable variations in their temperature, the influence of 
which is known. 
In those classes of animals which have little power of gene- 
rating heat, there are remarkable differences in the structure 
of their lungs, and in the composition of their blood, from the 
mammalia and aves. 
Respiration is one of the known causes which influences the 
temperatures of animals : where these organs are extensive, 
the respirations are performed at regular intervals, and are not 
governed by the will, the whole mass of blood being exposed 
to the atmosphere in each circulation. In all such animals 
living without the tropics, their temperature ranges above the 
ordinary heat of the atmosphere, their blood contains more of 
the red particles than in the other classes, and their muscular 
irritability ceases more rapidly after violent death. 
The respirations of the animals denominated “ cold-blooded,” 
