THE EAR 



171 



402. The ear drum is a tightly stretched membrane, which 

 is so constructed that it vibrates equally well with sounds of 

 different pitch. 



403. Between the ear drum and the inner ear there is a small 

 cavity which communicates with the pharynx through the 



',.,;/./;« 



Fig. 85. Fig. 87. 



Fig. 85. — Cross section of one turn of the cochlear spiral as it lies in position 

 in the long labyrinth. The organ of Corti (above the letter C) rests on the 

 basilar membrane and nerve fibres run out to the spiral ganglion .V. 



Fig. 86. — Part of the organ of Corti, to show the sensory cells and the nerve 

 fibres leading to the spiral ganglion. 



Fig. 87. — Diagram of the middle car of a mammal. E, K.xtornal auditory 

 passage, ending at the ear drum; /, internal ear; M , middle ear, opening into the 

 pharynx by the Eustachian tube E.T.; i, malleus; 2, incus; 3, stapes, fitting 

 into the oval window. 



Eustachian tube. This cavity is the middle ear. Its most 

 important parts are three small bones, the hammer, the anvil 

 and the stirrup, through which the vibrations of the car drum 

 are transmitted to the perilymph. The hammer is attached 

 to the ear drum, the stapes fits over the opening in the bony 

 labyrinth, and the two are connected by the anvil. In Birds 



