Mechano- and Eqitilihrinm-Heception 



507 



rapid stretch. Also, when adapted to stretch, the ending sometimes responded 

 to a tuning fork at one half or one third of the frequency of vibration.-" 



Fish detect pressure waves and ripples from a distant source by means of 

 skin receptors and lateral line organs. Blind fish snap at prey moving 5 to 

 10 cm. away, and by conditioned reflex techniques it has been shown that 

 minnows locate water jets and moving objects by the lateral line organs.^*'' 

 Electrical recording of the nerve impulses from catHsh lateral line nerves 

 shows that lateral line organs follow low frequency vibrations well but fail 

 to synchronize with the stimulus above about 100 cycles per second. ■^-' ■*"^' *^'' 

 In Fiinchdus, the lateral line discharge synchronizes at frequencies as high as 

 180 per second'" (Fig. 179 B, C). There is a continuous background of "spon- 

 taneous" activity in lateral line nerves which originates in the neuromasts, and 

 responses to mechanical stimulation are superimposed on this background. 

 The response in the lateral line when another fish swims by is shown in 

 Figure 179, A, and the lateral line must be very important in keeping fish 

 properly oriented when they swim in schools. The skin receptors of fish also 

 detect vibrations but have a higher threshold and are more important in 



Fig. 179. Responses in lateral line nerve of Fiindidus. A, Spontaneous activity with 

 three bursts of impulses as another fish swims by. B, Synchronized impulses at 75 and, C, 

 at 45 per second in response to sound. Signal from sound generator on baseline of 

 oscilloscope and impulses superimposed. From Suckling."' 



sensing contact stimuli. Impulses in ner\es from the barbels of catfish show 

 synchronized responses to ripples. ■*''■ ^■'' In those amphibians also v\hich pos- 

 sess lateral line organs CAinhlystoiua,'^''- Xenopus,''*'^ these sensory structures 

 are very efficient in localizing moving objects and in detecting fixed ob- 

 jects by reflected waves. 



Orientation to Contact Stinnili. Several kinds of direct responses to me- 

 chanical stimulation are known. Many animals, particularly those which 

 crawl, tend to aooreoate in crevices and along surfaces, a thigmotaxis. This 

 reaction provides protectix'e shelter and is easily overbalanced by other types 



