Sensory Discrimination : Hearing 117 



exhaustive history of which will be found in Parker's mono- 

 graph entitled "The Function of the Lateral-line Organs 

 in Fishes." The problem seems to have been solved by 

 Parker's own experiments. He first proved experimentally 

 that the canals played no part in responses to the following 

 stimuli : light, heat, salinity of the water, food, oxygen dis- 

 solved in the water, carbon dioxide, foulness of the water, 

 hydrostatic pressure, steady currents flowing through the 

 water, and sound. When, however, the water in the aqua- 

 rium was made to vibrate slowly, about six times per second, 

 the fish made certain characteristic reactions, differing 

 somewhat for the four or five species observed, but always 

 failing to appear when the lateral-line nerve was cut. Parker 

 concludes that "the stimulus for the lateral-line organs (a 

 water vibration of low frequency) is a physical stimulus inter- 

 mediate in character between that effective for the skin (de- 

 forming pressure of solids, currents, etc.) and that for the ear 

 (vibrations of high frequency), and indicates that these organs 

 hold an intermediate place between the two sets of sense organs 

 named" (309). The ear is thus regarded as actually derived 

 from the lateral-line canal, as this in turn was derived from 

 the skin. We may suppose that at least three different sensa- 

 tion qualities result from stimulation of the skin, the canals, 

 and the ear, where hearing can be shown to exist. 



38. Hearing in Amphibia 



Emergence from the water, on the part of adult Amphibia, 

 is accompanied by disappearance of the lateral-line canals, 

 and consequently of whatever sensations these mediate. In 

 the frog, the ear has a tympanic membrane lying at the sur- 

 face of the head. A single bone, the columella, with one 

 end against this membrane, lies across the middle ear. The 

 internal ear is not essentially different in structure from that 



