514 Com-parative Animal Physiology 



which are stimulated by noxious stimuh and give rise to impulses in small 

 myelinated fibers. These are to be distinguished from pressure endings of 

 the periodontal membrane, which show much variation in adaptation and 

 some of which follow very high frequency stimulation, even as high as 1200 

 cycles per second.'"'^ 



In the earthworm sensory cells converge at the subepidermal plexus, and 

 the afferent fibers of the segmental nerves are, therefore, secondary neurones. 

 Nevertheless the afferent impulses set up by tactile and proprioceptive 

 stimulation are large and fast compared with the impulses initiated by 

 noxious stimuli, particularly chemical.'^^ It is a general rule that pain im- 

 pulses are small and slow, in small fibers. 



EQUILIBRIUM RECEPTION 



Types of Equilibrium Receptors. One of the more primitive sense 

 organs controlling animal attitude is the gravity receptor or statocyst. In 

 principle a statocyst (Fig. 184) consists of a Huid-filled chamber, the bot- 



Fig. 184. Apical statocyst of a ctenophore. Note the centrally located structure, the 

 statolith, and the hairlike processes of the sensory epithelium. From Buddenbrock.^' 



torn or top of which contains a sensory epithelium. A solid or semisolid 

 body, the statolith, rests on or hangs from this epithelium and thus pre- 

 sumably activates the sense cells. If these sense cells exhibit moderately slow 

 adaptation, a steady barrage of nerve impulses at rather low frequencies 

 may be passing along the nerve fibers. Any change in position or weight 

 of the body resting on or hanging from the sensory epithelium would either 

 enhance or diminish this barrage of nerve impulses, thus signaling the co- 

 ordinating centers of the central nervous svstem of a change in attitude 

 of the organism. The pattern of motor reactions elicited by stimulation of 

 statocysts varies from animal to animal, for it depends on the extent and 

 characteristics of the reflex pathways involved, but many organisms react 

 in such a manner as to bring the statocyst and its contents back to its original 

 resting condition, thus effectively annulling the change in the statocyst 



