The abdomen is carefully inspected for the presence of dilated superficial veins or a caput medusae (varicose veins radiating from the umbilicus). If present, these are lightly palpated for the presence of a thrill. If a thrill is present, a venous hum is present. If the thrill is absent, a venous hum may or may not be present. Finally, a hum may be present when dilated veins are not seen. The hum is listened for by using light pressure with bell or diaphragm of the stethoscope. If detected, the hum can be obliterated by increasing the pressure of the stethoscope or by compressing the veins with the free hand. The abdominal venous hum, like the cervical venous hum, is a continuous roaring or whining noise, which may be localized to the abdomen or may radiate into the chest. Unlike the cervical venous hum, the response of the abdominal venous hum to change in position, respiration, cardiac cycle, or the Valsalva maneuver is unpredictable. For instance, the Valsalva maneuver obliterates the cervical venous hum but may increase or decrease the abdominal venous hum. It is essential that the abdominal venous hum not be confused with respiratory noises and bowel sounds. The problem of respiratory noises is solved by having the patient hold his breath. Bowel sounds can be quite confusing, but usually can be distinguished by their tinkling, changing, and intermittent qualities.