426 - Multicellular Animals, Especially Man 



REISSNER'S 

 MEMBRANE 



SCALAVESTIBULI 



AFFERENT 

 NERVE FIBERS 



SCALA TYMPANI 



Visceral Receptors. The interoceptors, 



which are localized in the internal organs, 

 play an important role in governing the ac- 

 tivities of the viscera. However, the intero- 

 ceptors of the body are not so well defined as 

 are the various exteroceptors, which have 

 been discussed. The sense of thirst appears 

 to originate in the throat, although the thirst 

 receptors have not been identified definitely. 

 A dearth of water in the digestive tract, and 

 in the body tissues generally, seems to act as 

 a stimulating agency that prompts the ani- 

 mal to seek and drink the necessary quanti- 

 ties of water. 



Hunger sensations originate in the wall of 

 the stomach. The empty stomach is swept by 

 a series of rhythmic muscular contractions, 

 and these hunger contractions exert a me- 

 chanical action upon the pressure receptors 

 in the gastric wall. Likewise the urge to 

 defecate and to urinate take origin from 

 mechanoreceptors in the walls of the rectum 

 and bladder respectively, as a result of ten- 



Fig. 23-9. Section through part 

 of the cochlea. The hair cells 

 are the essential receptors of 

 sound vibrations. 



sion that develops when these hollow organs 

 become distended by their contents. 



Nerve impulses from the interoceptors sel- 

 dom cross the threshold of consciousness, be- 

 cause such impulses are not usually transmit- 

 ted to the higher centers of the brain (p. 

 461). Nevertheless the visceral receptors are 

 most important, especially the receptors in 

 the walls of the blood vessels and heart, 

 which initiate reflexes controlling the blood 

 pressure and pulse rate. Moreover, the re- 

 ceptors in the walls of the lungs play a ver\ 

 important role in controlling the amplitude 

 of the breathing movements (p. 367). 



PERCEPTION OF SENSATIONS 



The sense organs merely discharge excita- 

 tions into the nervous system, and the recep- 

 tor cells are not directly responsible for any 

 sensation that may be experienced by the 

 stimulated individual. What sensation, if 

 any, may result depends not upon the activity 



