RESPONSIVENESS IN VERTEBRATES 



brain by way of the auditory nerve. How the auditory centers of the brain 

 translate these signals into the call of a bird, a symphony, or the sounds of a 

 city is unknown. Man can detect sounds ranging in frequency from 16 to 

 20,000 waves per second (middle C is 256 waves per second). Very loud 

 sounds are felt as well as heard and sometimes cause a sense of pain. 

 Exposure to intense sounds may lead to destruction of specific regions of the 

 basilar membrane and so to deafness for tones of the same frequency. 



Vertebrates become aware of chemical substances in their environment if 

 these substances become dissolved in the moist surface of the olfactory epithe- 

 lium or in the secretions of the mouth cavity. The nasal epithelium contains 

 the olfactory receptors which are actually neurosensory cells such as occur in 



Auditory nerve fibers 



Fig. 4.17. Crista of inner ear; semidiagrammatic. There is a sensory area, or crista, in the 

 ampulla of each semicircular duct (cf. Fig. 4.4). Movement of fluid in a semicircular duct, each 

 of which is located in a different plane, brings about a change of position in the cupula in which 

 extensions from the sensory or hair cells are embedded. Thus the hair cells are stimulated, and 

 an impulse passes over fibers of the auditory nerve toward the brain. Cristae are stimulated by 

 changes in rate of movement of the head and especially by rotational movement. The structure of 

 the macula, the sensory area in the utricle of the vestibular portion of the inner ear, is essen- 

 tially like that of a crista. However, the gelatinous material in which processes of the hair 

 cells are embedded is arranged in a layer known as the otolith membrane because it contains small 

 masses of calcium carbonate called otoliths. These respond to gravitational pull no matter what 

 the position of the head; the consequent stimulus of the hair cell makes us aware of the position 

 of the head even when it is not moving. There is a macula in the sacculus of the vestibule, but 

 its function has not been determined. 



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