Page:The American Cyclopædia (1879) Volume V.djvu/499

 CROCODILE 495 arise so far back, that they act in part upon the whole head, explaining the assertion made from the time of Aristotle to that of Cuvier, and at various times believed and disputed, that the crocodile has the ability to move both jaws ; when the lower jaw is fixed upon the ground, the action of the muscles may raise the whole head, and with it the upper jaw, other- wise immovable. The jaws have no lateral motion, and none from before backward, the articulation being a simple hinge joint. There are no cutaneous lips, the teeth being visible even when the jaws are closed. The teeth are numerous, conical, isolated, unequal in size, hollowed at the base, arranged in a single row, implanted by a true gomphosis in the substance of the maxillary borders, in special alveoli di- rected from before backward, and provided with a kind cf gum; the new teeth push up into the hollow of the old, and cause their ab- sorption; the new teeth are larger, but the same in number at all ages. The tongue is flat, wide, fleshy, and attached all around to the jaw bone ; it is not divided at the tip, and cannot be extended, being apparent only when the jaws are separated, and forming the floor of the mouth ; it cannot be used to seize or re- tain their prey, nor for respiratory purposes; it is for the most part smooth, except at the base, where irregularly contorted folds are prominent. The nostrils open at the end of the muzzle, near together, and may be closed by valves ; their cavity forms two canals, ex- tending along the cranium, and opening, not into the mouth, as in other reptiles and birds, but into the posterior fauces behind the soft palate, as in mammals ; the hyoid bone sends upward a rounded cartilaginous continuation, which can be made prominent at the will of the animal; the soft palate hangs down to meet this, by which the cavity of the mouth can be completely shut off from the fauces; by this arrangement, when the animal is under water, with only the tip of the nose in the air, and even with the mouth filled with water, respiration can be perfectly carried on ; and by the same mechanism the act of swallowing can be accomplished beneath the surface. Un- like the saurians, this family have the external opening of the ears protected by two folds of the skin, resembling lids, by which the meatus can be closed ; the opening is just be- hind the eyes. The eyes are very small, and provided with three lids, an upper and lower, with a third or nictitating membrane moving transversely, transparent, and evidently de- signed to protect the cornea and permit vision under water ; the pupil is a vertical slit, and the crystalline lens almost spherical. The anterior limbs have five toes, the external two without nails; the posterior limbs are four-toed, more or less webbed, the external one without a nail ; the limbs are so short that they barely raise the body from the ground, and are almost at right angles to the spine ; their gait is, there- fore, slow and awkward. The tail is longer 237 VOL. v. 32 than the trunk, flattened on the sides, sur- mounted with crests continued from the back, and serrated below ; the powerful muscles of the dorsal region are carried to their greatest development in the sides of the tail, which is the principal organ of locomotion in the water. The stomach is muscular, but in no way re- sembling the gizzard of a bird ; in this cavity are frequently found stones and pieces of wood, which were once supposed to be swallowed in- tentionally to assist in triturating the food, or for the purpose of distending the stomach du- ring the season of hibernation which some of them undergo; it is altogether probable that such foreign bodies have been accidentally swallowed during the repasts of the voracious animal. The lungs consist of three principal cavities, communicating freely with each other ; the walls are divided into innumerable cells, the fleshy compartments of which form a very intricate network, resembling the columnae car- neee of the heart; when fully expanded, they will contain a large quantity of air. The most interesting organ is the heart, as it shows an approach to, and as it were the connecting link with the birds. In reptiles generally the heart consists of three cavities, a ventricle and two auricles; the ventricle receiving both arterial and venous blood, and sending this mixed fluid over the system at the same time that it sends to the lungs blood of which a portion has just been received purified from them. In the croc- odilians the ventricle has a complete division into right and left, and the circulation is so arranged that while the head and anterior half of the body receive pure arterial blood when the animal is in the air, the posterior half receives a mixed arterial and venous blood ; the mingling of the two bloods taking place, not in the heart itself, but by an opening between the two aortas, a fact unknown to naturalists till the time of Meckel and Panizza. The ordinary course of the circulation would be through the venss cavaa to the right auricle, thence to the right ventricle ; from this more than half of the venous blood goes to the lungs by the pulmonary artery, the rest being dis- tributed to the lower extremities through the left or venous aorta ; from the lungs the pure blood comes to the left auricle, thence it passes to the left ventricle, and then by the right or arterial aorta to the head and anterior extrem- ities and body generally, after mixture with the venous blood. In the common circulation, or when the animal is in the air, there would probably be but a trifling, if any, mixture of the bloods through the opening in the aortic wall, and during the contraction of the ventri- cles the pressure of the valves of the aortse against the opening would prevent the min- gling ; but during the diastole of the ventricles, when the valves close to prevent regurgitation into the heart, the aortic opening would be free, and the bloods could mix in whichever direc- tion the pressure was the strongest ; the open- ing, however, performs its special function after