Mechano- and Equilihrium-Reception 527 



stimulation, as in a worm crawling in a crevice. In general tactile receptors 

 are most abundant in those regions of an animal where there is most likely 

 to be contact with extraneous objects. Vibration receptors are di\'erse— 

 lateral line organs of fish, sensilla attached to the exoskeleton of insects, 

 tendon and muscle receptors of mammals. A change in plane is signaled 

 by equilibrium receptors or by a combination of statocysts, tension receptors, 

 and eyes. 



Several questions deserve investigation. What is the mechanism by which 

 deformation excites a cell? A continuous "spontaneous" activity is found in 

 sensory nerves from fish lateral hues, in ampullae of Lorenzini of elasmo- 

 branchs (which are said not to be mechanoreceptors), and in vertebrate laby- 

 rinths. Are these impulses dependent on some very slight sensory stimulation 

 or do the sense organs contain cells which discharge spontaneously like some 

 central nervous system neurones? Two kinds of adaptation of mechano- 

 receptors are recognized, primary adaptation during continued steady de- 

 formation, and secondarv adaptation during repeated deformation of end- 

 ings which adapt rapidly to one deformation. What are the mechanisms 

 of these two types of adaptation? There is evidence that secondary adapta- 

 tion of frog tactile receptors results from an accumulation of potassium around 

 them; does this account for the similar adaptation of vibration receptors? 

 Another question concerns the action of the receptors of hydrostatic pressure 

 in keeping deep-sea and surface fish at the proper ocean depth. 



Mechanical stimuli (except for hydrostatic pressure) can hardly be con- 

 sidered as limiting animal distribution, but they are important in keeping 

 animals in position for their normal life activities. Equilibrium receptors are 

 found in manv animals and appear to be particularly important for animals 

 which are free swimming, for example, jellyfish, elasmobranchs, and 

 teleosts, and in flying animals, particularly birds and certain flying insects. 

 Equilibrium receptors are also especially important when the center of 

 gravity is well above substratum as in many birds and mammals. 



REFERENCES 



1. Adrian, E. D., Mechanism of Nervous Action (1935). Univ. of Pennsylvania 

 Press. 103 p. 



2. Adrian, E. D., Cattell, M., and Hoagland, H., J. Physiol. 72:377-391 (1931). 

 Sensory discharges in single cutaneous nerve fibers, frog. 



3. Adrian, E. D., and Umrath, K., ]. Physiol. 68:139-154 (1929). Sensory impulses 

 from pacinian corpuscles. 



4. Best, C. H., and Taylor, N. B., Physiological Basis of Medical Practice (1943). 

 Baltimore, Williams and Wilkins. pp. 1346-1363. Mechanoreception. 



5. Bishop, G. H., /. Neurophysiol. 6:361-382 (1943). Responses to electrical stimu- 

 lation of sensory units of skin. 



6. Bishop, G. H., J. Neurophysiol. 7:71-80 (1944). The peripheral unit for pain, 

 man. 



7. Black, V. S., Biol. Bull. 95:83-93 (1948). Gas changes in swim bladder of Fun- 

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8. Blair, H. A., Am. J. Physiol. 114:586-593 (1936). Mechanical stimulation of 



nerve. 



9. Breuer, J., Wien. tned. Mrb. 4:72-124 (1874), ifoid. 5:87-156(1875). Function 



of the labyrinth, vertebrates. 

 10. Breuer, J., Pfliig. Arch. ges. Physiol. 48:195-306 (1891). The function of the 

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