Circulating System.—The heart in the Chelonia is composed of two auricles, and one ventricle with two unequal chambers which communicate together. The blood of the body enters into the right auricle and that of the lung into the left ; but both these modifica tions of blood mingle more or less in passing by the ventricle.
Respiratory System.—Cuvier remarks that the quantity of respira tion in Reptiles is not fixed, like that of Mammals and Birds, but varies with the proportion of the diameter of the pulmonary artery compared with that of the aorta. Thus, ho observes, the Tortoises and the Lizards respire much more than the Frogs.
The lungs are of great extent, and placed in the same cavity with the abdominal viscera. We have seen that the thorax is immoveable, in the greater number at least, and the inlaid fixed ribs can give no assistance in respiration in the full-grown normal forms. It is there fore by the play of the parts about the mouth that the Che/onia respire, and here the complicated os hyoides is called into prominent action. • The jaws are closed, and the animal alternately elevates and depresses time os hyoides; the first movement lets the air enter by the nostrils, and the tongue then closing their interior aperture, the second movement compels the air to penetrate into the lungs. in short, the Tortoises swallow or gulp down the air necessary for their respiration like Frogs.
John Hunter, in his a MS. Catalogue,' observes that the vessels of the lungs of those animals whose whole blood passes through them are confined to the lungs, and lungs only, as distinctly as if the lungs were a separate animal; but this, he adds, is not the case with the Amphibia," for," says he, " we find the vessels of the lungs of the Turtle communicate with those of other parts, such as the vessels of the (esophagus, which shows that the blood of that part is not so perfect in them as ittothera. From this it must appear that the lungs are not of that consequence in this class of animals that they are in the more perfect, for the lungs themselves appear to share in common with the other parts. Some of the blood which just came from the lungs returns back again to them, which would appear to answer no purpose; and on the other hand a considerable quantity of the blood which had undergone the general circulation (and therefore would appear to require refinement) just returns through the sanae course. It would
appear from this admixture that it was not necessary that the whole of the blood should have undergone a thorough change for its greatest motion ; yet we do not see why the lungs should have a part of their blood of the perfect kind. The cells of the lungs of the A.mphibie seem to increase in size, the farther from the trunk or trachea, so that the trachea and its ramifications bear no proportion between them and the cells." Brain, Nervous System, and Senses.—In the Chelonia generally, the vertical height of the capacity of the cranium is greater than in the other Reptiles; but in the Sea-Tortoises, or Turtles, the mass of the encephalon does not entirely fill it, and the highly vaulted bones mire rather destined to serve as solid points of resistance to the upper beak, and to the powerful action of the muscles which act upon the lower jaw. The mass of the encephalon is lass elongated and more compact than in the serpents. hiojauus, in his work on the Anatomy of the F:uropean Etnys,' has shown that the great sympathetic or ganglionic series of nerves exists in that reptile nearly as it does in the other Vertebrate ; that on the one hand it has sympathetic rela tions with the encephalic and vertebral nerves, and that on the other it makes a communication between the two lateral and symmetrical parts of the body, at the same time that its filaments are distributed and intermingle in numerous plexuses round the principal arteries destined to the nutrition of the internal viscera. Elaborate illustra tions of the Nervous System, and especially of the great Sympathetic of the Hawk's-Bill Turtle, have been published by Mr. Swan, in 116 Comparative Anatomy of the Nerves,' 9to., I8343.
Here we must notice the experiments of Heti, which were perhaps more cruel in appearance than in reality. Most are familiar with the length of time that a turtle will move after its head is off, and the snap of the jaws which the severed head will give ; but there is reason for believing that there is more of irritability than sensation in such motions; and the state of Rail's tortoises must have been analogous.