Responses of Higher Animals: The Receptors - 425 



COCHLEA 



or„,o,n,„,r> SCALA MEMBRANOUS SCALA 

 SEM J? R .^ R VESTIBULk CANAL .TYMPANI 

 CANALS 



EUSTACHIAN 

 TUBE 



VESTIBULE 



Fig. 23-8. Structure of the human ear. The shaded portions with heavy outlines 

 represent bone; the lighter lines represent membranous structures. 



with fluid and lined with hair cells. The sac- 

 cule and utricle appear to function more or 

 less like statocysts, but the semicircular canals 

 are concerned with the perception of rota- 

 tional movement, not of position. The differ- 

 ent semicircular canals lie in different planes, 

 each at right angles to the others, and when 

 the body starts to move in a given direction, 

 inertia displaces the fluid in some one of the 

 canals, exciting some local group of the hair- 

 bearing receptor cells. If the movement stops, 

 the momentum of the fluid displaces the 

 fluid in an opposite direction, which excites 

 some other group of hair cells. 



In fish and other lower Vertebrata, the 

 labyrinth is mainly an organ of equilibrium; 

 but in land vertebrates part of the labyrinth 

 develops into the cochlea, the essential organ 

 of hearing (Fig. 23-8). The cochlea contains 

 a large number of hair-bearing sensory cells 

 (Fig. 23-9), which are stimulated by sound 

 vibrations, transmitted from the external air, 

 through the tympanic cavity, to the fluid in 

 the cochlea. Perception of the pitch and 

 quality of sounds depends on the fact that 

 the different hair cells of the cochlea are 

 stimulated by vibrations of different fre- 



quencies. Probably insects are the only other 

 animals that possess specialized organs of 

 hearing, but the ears of insects vary widely 

 as to structure and position in the different 

 species. 



Proprioceptors. The perception of move- 

 ment and position of the body as a whole 

 is localized in the labyrinth, but each sepa- 

 rate muscle and tendon is equipped with re- 

 ceptors, called proprioceptors, which play an 

 essential role in coordinating the complex 

 movements of the individual body parts. The 

 proprioceptors of the muscles (Fig. 23-1 D) 

 and tendons (Fig. 23-1C) are sensitive to the 

 changes of tension, and when a muscle is 

 brought into play, its proprioceptors continue 

 to discharge a series of excitations along the 

 sensory fibers of its nerve supply. Thus each 

 gradation of tension serves to condition the 

 further activity of the muscle and bring it 

 into synchrony with the activities of other 

 muscles. Without the proprioceptors, skilled 

 movements would not be possible; and be- 

 cause of the efficiency of the proprioceptors 

 it is possible to perform skilled movements 

 without the guidance of the eye — as, for ex- 

 ample, the tying of a knot behind one's back. 



