![]() ![]() Just caudoventral to the thymus is the thyroid. It does not involute when the animal matures as it does in higher vertebrates. The single or double lobed thymus is found craniolateral to the thyroid gland, closely associated with the vagus. It has both endocrine and exocrine functions much the same as in mammals. The pancreas is found caudal to the gall bladder on the mesenteric border of the duodenum. It is usually tightly adhered to the pancreas, and the two organs collectively are often referred to as the splenopancreas. The spleen is a small, spherical, reddish organ located between the gall bladder and the pancreas. There is an extensive network of perivascular lymph channels around the major vessels and perivisceral lymph spaces which drain the viscera. However, the lymphatic system in reptiles is complex. Reptiles, except the snapping turtle, do not have lymph nodes. In the turtle the renal portal system receives veins from the carapace, the musculature posterior to the kidneys and the external iliac veins. In the lizard the caudal tail vein and the internal and external iliac veins all feed through the kidneys before returning to the heart. In the snake the parietal veins from the body wall and the caudal vein from the tail pass through the kidneys before anastomosing with the ventral abdominal vein. There is an incomplete ventricular septum which allows the heart to function as a four chambered heart. The three chambered reptilian heart is composed of two atria and a large ventricle. These are assisted by other muscles of the trunk and abdomen, as well as smooth muscles in the walls of the lungs themselves. Breathing (inspiration and expiration) is accomplished principally by the intercostal muscles. ![]() The right bronchus terminates in the functional right lung.Īll reptiles, except the crocodile, lack a diaphragm. The size and functional capacity of this left lung varies from species, and can be complete in some of the water snakes where it is used for hydrostatic purposes. In the snake the trachea branches into a short left bronchus which terminates in a vestigial left lung. In the lizard and turtles the trachea bifurcates into two bronchi which then enter the left or right lung. The trachea usually terminates just dorsal to the heart. These rings are complete in the turtle and the crocodile, and incomplete in the lizard and snake. The trachea is usually long and is supported by cartilaginous rings. Snakes are able to extend their glottis out the side of their mouth while they are eating to allow for respiration. It forms a vertical slit in the closed position. Unlike mammals, the reptile glottis is always closed unless it is taking a breath. There is a small opening caudal to the tongue called the glottis. Venomous snakes substitute fangs for the maxillary teeth. There are only two rows on the lower jaw, one attached to each mandible. Non-venomous snakes have four rows of upper teeth: two rows on the maxilla and two rows on the palatine-pterygoid bones. Turtles do not have teeth, but instead, they have a horny beak which they use for biting. The teeth of snakes and lizards are both acrodont (attached to the bone) and polyphydont (capable of having several sets throughout life). The flicking tongue picks up minute scent particles in the air and places them in direct contact with this organ. Its paired openings are just rostral to the choana. Snakes and some lizards have a special sensory structure called the vomeronasal or Jacobson's organ. There are a few species of lizards which lack this feature. Lizards and turtles lack external pinnae, but most have a conspicuous tympanic membrane. They do have an internal ear which functions in detecting motion, static position and sound waves which travel through the ground. Fortunately, all legless lizards do! The snake lacks not only the external ear, but also the middle ear cavity, tympanic membrane and eustachian tube. But, to complicate matters, not all lizards have ears. ![]() Occasionally this spectacle will not come off with the skin, and results in a retained eye cap.Ī second obvious difference between snakes and legless lizards is that snakes do not have ears. When a snake goes through ecdysis, or shedding, it will slough this spectacle with its skin. The snake eye is protected by a transparent scale called the spectacle. Lizards and turtles do have functional eyelids (with some exceptions such as some members of the gecko family). Thus, it is important to be able to distinguish a snake from a legless lizard. There are no snakes with legs, but there are lizards without legs. In general, all reptiles are covered with scales. ![]()
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