two pleurae of the lungs, is detected by an x-ray of the chest, showing fluid collecting in the angle between the ribs and diaphragm. an x-ray may also be used to reveal a pneumoperitoneum, in which there is gas in the abdomen. an x-ray may also be used to check for herniation. significance in strength training the adoption of a deeper breathing pattern typically occurs during physical exercise in order to facilitate greater oxygen absorption. during this process the diaphragm more consistently adopts a lower position within the body's core. in addition to its primary role in breathing, the diaphragm also plays a secondary role in strengthening the posture of the core. this is especially evident during deep breathing where its generally lower position increases intra-abdominal pressure, which serves to strengthen the lumbar spine. the key to real core stabilization is to maintain the increased iap while going through normal breathing cycles. the diaphragm then performs its breathing function at a lower position to facilitate a higher iap. therefore, if a person's diaphragm position is lower in general, through deep breathing, then this assists the strengthening of their core during that period. this can be an aid in strength training and other forms of athletic endeavour. for this reason, taking a deep breath or adopting a deeper breathing pattern is typically recommended when lifting heavy weights. other animals diaphragm and pleural cavities in amphibian (left), bird (center), mammal (right). a, mandible; b, genio-hyoid; c, hyoid; d, sterno-hyoid; e, sternum; f, pericardium; g, septum transversum; h, rectus abdominis; i, abdominal cavity; j, pubis; k, esophagus; l, trachea; m, cervical limiting membrane of abdominal cavity; n, dorsal wall of body; o, lung; o', air-sac. the existence of a membrane separating the pharynx from the stomach can be traced widely among the chordates. thus the model organism, the marine chordate lancelet, possesses an atriopore by which water exits the pharynx, which has been claimed (and disputed) to be homologous to structures in ascidians and hagfishes. the tunicate epicardium separates digestive organs from the pharynx and heart, but the anus returns to the upper compartment to discharge wastes through an outgoing siphon. thus the diaphragm emerges in the context of a body plan that separated an upper feeding compartment from a lower digestive tract, but the point at which it originates is a matter of definition. structures in fish, amphibians, reptiles, and birds have been called diaphragms, but it has been argued that these structures are not homologous. for instance, the alligator diaphragmaticus muscle does not insert on the esophagus and does not affect pressure of the lower esophageal sphincter. the lungs are located in the abdominal compartment of amphibians and reptiles, so that contraction of the diaphragm expels air from the lungs rather than drawing it into them. in birds and mammals, lungs are located above the diaphragm. the presence of an exceptionally well-preserved fossil of sinosauropteryx, with lungs located beneath the diaphragm as in crocodiles, has been used to argue that dinosaurs could not have sustained an active warm-blooded physiology, or that birds could not have evolved from dinosaurs. an explanation for this (put forward in 1905), is that lungs originated beneath the diaphragm, but as the demands for respiration increased in warm-blooded birds and mammals, natural selection came to favor the parallel evolution of the herniation of the lungs from the abdominal cavity in both lineages. however, birds do not have diaphragms. they do not breathe in the same way as mammals and do not rely on creating a negative pressure in the thoracic cavity, at least not to the same extent. they rely on a rocking motion of the keel of the sternum to create local areas of reduced pressure to supply thin, membranous airsacs cranially and caudally to the fixed-volume, non-expansive lungs. a complicated system of valves and air sacs cycles air constantly over the absorption surfaces of the lungs so allowing maximal efficiency of gaseous exchange. thus, birds do not have the reciprocal tidal breathing flow of mammals. on careful dissection, around eight air sacs can be clearly seen. they extend quite far caudally into the abdomen.