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, and insecta, 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 atmosphere, 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 principal 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 generating 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 irritabilhy ceases more rapidly after violent death.
The respirations of the animals denominated "cold-blooded,"
