 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 they do not possess a diaphragm. So, both are abdominal and thoracic cavities. However, pressure differences between these two regions do exist. That is why there is some indication of functional separation of these two cavities. The difference between these two cavities plays an important role in the mechanism of 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 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 of diaphragm in expanding chest cavity. Principally, 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, lungs increase in pressure and air passively moves out. Now, we shall discuss the breathing mechanism in Turtles and Tortises. Turtles' ribs are a shell part, so 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 abdominal cavity are pushed upward. The result is that the lungs are compressed and air is exhaled. The opposite of this is inhalation. When abdominal wall muscles contact the viscera down like this, it creates a negative pressure on the lungs and air is drawn into the lungs.