Fish characteristically breathe continuously with a respiratory rhythm originating in the brain. This neural generator has a diffuse and, as yet, poorly defined location within the medulla oblongata. Its output to gill musculature is modulated by inputs from the midbrain and stabilized by feedback from gill mechanoreceptors. Fish also may breathe episodically and even arrest rhythmic breathing when ram-ventilating their gills during forward swimming. During periods of high oxygen demand, feeding muscles can be recruited into the respiratory cycle. These same muscles are used to gulp air by air-breathing fishes. The respiratory rhythm generator for air breathing has yet to be clearly defined.