A MORPHOGENETIC STUDY OF THE RESPIRATORY SYSTEM OF THE NILE CROCODILE Crocodylus niloticus (Laurenti, 1768)

摘要

The Nile crocodile is found in a number of environments which shows that the crocodile is versatile in its habitat as selection is based on environmental pressures. The main mechanism by which respiration occurs in the Nile crocodile is through an aspiration pump which is general for reptiles. In crocodiles, known as the hepatic piston pump, the liver assists the aspiration pump by acting as a piston to ventilate the lungs. The liver is located posterior to the lungs and pulls the lungs posteriorly by the diaphragmatic muscles which are derived from the abdominal muscles. The diaphragmatic muscles protrude forwards from the pelvis and gastralia to the posthepatic septum. The posthepatic septum being a thin sheet posterior to the liver. The diaphragmatic muscles contract and pull the liver posteriorly so as to increase the lung volume and decrease the pressure of the lungs allowing atmospheric air to flow in. During exhalation, these movements are reversed. The diaphragmatic muscles relax and the liver is positioned against the lungs. Air is then expelled because there is an increase in pressure on the walls of the lungs. This system allows for the lungs to hold air for longer, allowing for longer dive time as crocodiles spend much time in the water. Much work has been done on the gross morphology but not much attention has been given to the fine morphology. The Nile crocodile was the first of crocodiles which makes it a good candidate to study the respiratory system changes of vertebrates.