[Illustration: Skeleton of Tortoise. A, superior maxilla; B, inferior maxilla; C, ossiculum auditus; D, os hyoides; E, cervical vertebræ; F, dorsal vertebræ; G, sacrum; H, caudal vertebræ; I, dorsal ribs; K, marginal scales; N, scapula; O, coracoid bone; P, os humeri; Q, radius; R, ulna; S, bones of the carpus; T, metacarpal bones; U, digital phalanges; V, pelvis; W, femur; X, tibia; Y, fibula; Z, tarsus; Æ, metatarsus; A.V., phalanges of the foot. ] But how comes it, the reader may ask, that respiration, which pours a warm current through our veins, fails in raising the temperature of the turtle's blood? Without entering into a lengthened description of the human heart, I shall merely observe that it consists of two halves (each half being again subdivided into two separate chambers), and that the right half, which receives venous blood and pours it into the lungs, is completely separated by a partition from the left half, which receives arterial or aërated blood from the lungs, and propels it into every part of the body. Thus the two different kinds of blood are completely separated, so that an _unmixed_ venous blood flows into the lungs, where it is converted by the oxygen of the air into arterial blood. But this connection, like most chemical processes, takes place under an evolution of heat, which is so considerable that our internal temperature constantly maintains itself at the height of 98° F. [Illustration: Theoretic Representation of the Circulation in Mammals and Birds.] [Illustration: Theoretic Representation of the Circulation in Reptiles.] But the turtle's heart is differently formed, consisting, as the annexed theoretic representation shows, of but one ventricle and two auricles, so that a _mixed_, or only half aërated blood circulates throughout the body, which naturally produces a torpidity of the whole vital process. Besides, the lungs of the reptiles are incapable of aërating so great a quantity of blood as ours, as their cells are much larger, thus offering less surface to the action of the air; and finally, the ribs of the turtles being immovable, they are incapable of extending the lungs, so that the animal is absolutely obliged to swallow the necessary supply of air, and to pump it, as it were, into the lungs, by contracting the muscles of the throat. Thus we see that every precaution has been taken to reduce respiration to a low standard, and prevent the evolution of heat. With this indolence of its cold-blooded circulation, the whole nature of the animal is in harmony; the bluntness of its senses, its want of intelligence, its slow movements, and its long endurance of hunger, thirst, and want of air. It leads but a drowsy dream-like existence, and yet, we may be sure, it is far from unhappy, for all its functions and organs agree perfectly one with the other, and when concord reigns, enjoyment of some kind must exist.