362 



THE MAINTENANCE OF THE INDIVIDUAL 



developed. In the land vertebrates amplifying devices are developed 

 in the form of a vibrating ear drum or tympanic membrane beneath the 

 skin, and a chain of middle ear hones that transmit the vibrations to 

 the inner ear where the sensitive receptor-cells are located. Thus 



A cross section of the coiled cochlea which contains the or^an of Corti in which 

 the sensitive hair cells are located. The scala media is filled with fluid endolymph 

 which is separated from the fluid perilymph of the scala vestibuli by Reissner's 

 membrane. Vibrations of the ear drum are transmitted throuKh the middle 

 ear bones which cause the vibration of a membrane at one end of the scala 

 vestibuli, thus disturbing the perilymph in the scala vestibuli. How are the hair 

 cells stimulated ? 



there is gradually developed an elaborate mechanism by which vibra- 

 tions are transmitted and amplified through the ear drum and the 

 three bones of the middle ear to the spirally coiled portion of the inner 

 ear, or cochlea, where the receptor-cells are located. These essential 

 cells receive stimuli which are carried by branches of the auditory 

 nerve to the brain for interpretation. 



Cutaneous Sense Organs 



There remains for consideration that diverse group of sense organs 

 located in the integument. In fishes, the tactile sense consists princi- 

 pally of pressure receptors, which are usually concentrated along the 



