 Dear students, in this topic we shall discuss the mechanism of lung ventilation in reptiles. Dear students, the snakes and lizards and crocodilians have ribs which form a thoracic cage around the lungs, just like mammals. However, in tortoises and turtles, the ribs are fused as a part of the rigid shell. So, these two groups are tortoises and crocodilian snakes and lizards. In these two, the mechanism of breathing is different. Reptiles have ribs but do not possess a diaphragm. So, both are abdominal and thoracic cavities are not different from each other. However, pressure differences between these two regions do exist. That is why there is some indication of functional separation of these two cavities. This difference between the two cavities plays an important role in breathing. We shall discuss in detail the breathing mechanism of snakes and crocodiles. First, we are going to discuss inhalation. During inhalation, ribs move posteriorly. They cause the expansion of thoracic cavity. This expansion reduces pressure in the lungs. As a result, air draws in passively into the lungs, passing through the open airs and glottis. The mechanism is similar to many other mammals. That is why there is no role in expanding the chest cavity. Principally, the chest cavity is expanded. Pressure on the lungs is reduced and air is inhaled. During exhalation, air is expelled out due to the elastic recoil of lungs and compression of thoracic cavity. The compression of thoracic cavity has that occurs due to forward movement of ribs and body wall. As a result of compression, the lungs increase in pressure and the air passively moves out. That is expiriation. Now we shall discuss the breathing mechanism in Turtles and Tortises. Because the ribs of turtles are shell-part, the forward-backward movement of body wall is not possible. The exhalation in these animals takes place by the contraction of abdominal wall muscles, which force the viscera or visceral organs upward. The organs of the abdominal cavity are pushed upward. The result is that the lungs are compressed and the air is exhaled. The opposite of this is inhalation. When the abdominal wall muscles contact the viscera down, it creates a negative pressure on the lungs and the air is drawn into the lungs.