SENSORY ORGANS AND RECEPTION 



345 



excepted), and usually closed in whalebone whales. Recent experiments 

 show that it is an efficient conductor of vibrations (50, 59). 



Tests on dolphins (Tursiops truncatus) in captivity reveal that these 

 animals possess a surprising range of auditory sensitivity, extending from 

 0-1 to at least 120 kc/s. When brief sound pulses (500-80,000 c/s) were 



Fig. 8.28. Records of Discharge in the Hyomandibular Nerve 

 of the Ray during Perfusion of the Hyomandibular Canal 



The white line is perfusion signal: upward displacement signals tailward perfusion; 

 downward displacement, headward perfusion. Time signal in seconds. The records show 

 the activity of the preparation at intervals of 1 min, when a headward perfusion of 1 min 

 duration (ab) was followed by a tailward perfusion of 1 min duration (de), with a rest 

 interval of 2 min between perfusions. A strong burst of large potentials accompanies 

 headward perfusion; the response to tailward perfusion is weak. (From Sand (127).) 



projected through the water, the dolphins responded by abrupt changes in 

 locomotion. Low frequencies (100-500 c s) produced greater locomotory 

 disturbance. Low-frequency vibrations possibly are perceived through 

 tactile stimulation of the skin, but the capacity to perceive high frequencies, 

 up to 60,000 c/s above the threshold for man, is probably mediated through 

 the inner ear. Dolphins also display comparable responses to recordings 

 of their own noises (56, 82, 102, 133, 134). 



