An involuntary opening of the mouth, which is accompanied by marked dilatation of the pharynx, a characteristic distortion of the face and usually stretching of the limbs. The cause and function of yawning are quite obscure. It is usually a sign of drowsiness or boredom, but it may be the result of raised concentrations of CARBON DIOXIDE (CO2) in the blood – the physiological aim being to cut the amount of CO2 and raise the level of oxygen in the blood.