Next in importance to the brain are the pneumatic perforations in the bones. They are seen in the lower jaw, the quadrate bone, in the whole of the vertebral column, in all the bones of the fore-limb, excepting one or two fragments, in the scapula and coracoid, in the os innominatum, in the femur and in the tibia. In such of the bones as can be compared, the pneumatic perforation is usually situated in Birds as it is in Pterodactyles. In Birds the bones are filled with air through these perforations, and as a principle the greater the motion of the animal, the greater is the number of bones filled with air. This air is received from the air-sacs which receive it from the lungs, and return it through the lungs again. There is thus in birds a sort of supplemental lung-system, which circulates air through the body. Nothing of the kind exists in any other class of animals. The respiratory system in birds is more perfect and complex than in the other vertebrata, and, as a result, the temperature of the blood on the whole is hotter. In Pterodactyles the reticulate character of the perforations proves that they were pneumatic, and supplied the bones with air. The fact that the bones were supplied with air, necessitates an elaborate system of air-sacs to furnish the supply. And the existence of these air-sacs speaks incontestably to bronchial tubes opening on the surface of the lungs to supply them, and to the existence of lungs essentially like those of birds. The outward and backward direction of the coracoid bones may indicate that the lungs were larger than in a bird. The circulation of air through the bird's body has relation to rapid motion through the atmosphere, which necessarily produces more rapid respiration than would comparative quiescence. The same inference must be applied to the Pterodactyles. But rapid respiration only means more rapid oxidation of the blood, and conversion of the purple cruorine into scarlet cruorine,--that is, the conversion of venous blood into arterial blood. And if venous blood is converted into arterial blood by a lung-apparatus like that of a bird, and with a rapidity like that in a bird, there must be a circulation of the blood as rapid as in birds. Such a circulation is only maintained by a heart with two auricles and two ventricles. Therefore Pterodactyles had the heart like that of birds and mammals. Now, since the temperature of the blood is chiefly dependent on respiration and circulation, and Pterodactyles had respiratory and circulatory organs which in living animals produce hot blood, it results that they were hot-blooded animals.