Mechano- and Equilihrinm-Reception 509 



increases the rhythmic activity in the lateral line nerve, whereas tailward 

 How reduces or stops the spontaneous rhythm.'^"' Whether the lateral line 

 is used in rheotaxis in Rnja is not known. 



Anemotaxis or orientation in air currents is seen in flying animals such as 

 some insects. In flies the antennal sense organ, Johnston's organ, is sensitive 

 to wind velocity and elicits postural changes according to speed of air flow.-*"'" 



When an animal is in contact with a substratum there is continuous asym- 

 metric stimulation of mechanoreceptors. Absence of ventral surface stimula 

 tion may initiate righting reactions as in snails, starfish, and cockroaches, and 

 removal of the substratum initiates flight in many insects. Similarly, lifting 

 of the sucker of a leech initiates swimming. Apparently in the absence of 

 other stimuli, stimulation by contact normally inhibits locomotor reflexes. 



Hydrostatic pressure is applied uniformly, and most of the effects of hydro- 

 static pressure or atmospheric pressure are respiratory rather than sensory. 

 One of the familiar eff'ects of rapid changes in hydrostatic pressure is in the 

 amount of nitrogen dissolved in the blood and the bubbles which appear on 

 decompression. Pressures of the order of several hundred atmospheres cause 

 spectacular decrease in protoplasmic viscosity, and many animals are killed 

 fairly rapidly at about 400 atmospheres. Yet fish and deep-sea invertebrates 

 live at ocean depths of two miles, where the pressure exceeds 300 atmos- 

 pheres.^" When deep-sea animals are brought to the surface they die, and, 

 conversely, surface animals cannot survive long at pressures corresponding to 

 the deep sea. The physiological adaptations permitting survival at high pres- 

 sures are not understood. However, fish do tend to remain within a certain 

 pressure range, and they do this in part by varying the amount of gas in the 

 swim bladder. A sudden decrease in pressure by a fraction of an atmosphere 

 results in an increase in gas content in the swim bladder (guppies'-O- In 

 physoclistous fish, that is, in those lacking a duct from gas bladder to esoph- 

 aous, the gas must be secreted into the bladder from the blood and the ef- 

 fect is to maintain the fish at a constant level.*'*^ Similar increases in bladder 

 oas content occur when Fnndtdus is transferred from sea water to fresh wa- 

 fer.^ In some phvsostomes (fish with open swim bladders) certain anterior 

 vertebrate form a chain of bones, the Weberian ossicles, which transmit pres- 

 sure to the inner ear from the swim bladder (p. 486, Ch. 13), and removal 

 of which prevents normal reactions to a pressure drop.^'^ An increase in swim- 

 bladder pressure in the carp elicits a strong response of all fins, cardiac de- 

 pression, and initial stimulation, followed by depression of respiration-all 

 eflfects which cease when the autonomic nerves to the swim bladder are cut.-*-' 

 The gas bladder of fish is an effector containing sensory mechanisms which 

 aid in maintaining fish at constant hydrostatic pressure. 



Tactile Sense. Ability to locate contact stimuli may be very precise. The 

 cleaning habits of most animals are well known-molluscs, insects, and many 

 others remove irritating particles with great precision. Tactile sense in man 

 has been most studied by the use of needles or flexible prods of different 

 weights. On the skin the sensitivity of hairs is greater than that of cutaneous 

 corpuscles; the sensitivity of shaved skin differs greatly, the sensitivities in 

 various areas decreasing in the following order: forehead, nose, finger tip, 

 back of finger, back of hand, abdomen, back of forearm, loin. The ability to 

 discriminate two points is not the same as the sensitivity series, minimal 



