422 HUMAN PHYSIOLOGY 



poral bone there are found two parts, one of which is called, collectively, 

 the semicircular canal region, the other the cochlea or organ of hearing. 

 Both of these organs consist of membranous bags lying in a fluid which 

 partially fills the bony cavity which incloses them. These membranous 

 structures themselves also contain a fluid. The semicircular canals are 

 connected with the cochlea on one side, and are separated from the middle 

 ear only by a membrane and the fluid which surrounds them. There are 

 three semicircular canals, delicate membranous bags lying in a watery 

 fluid and surrounded by bone. 



It has been discovered by experimenting with fish in which the semi- 

 circular canal region forms the chief part of the ear, that it has to do with 

 the equilibrium or balancing of the body. We gain our knowledge of our 

 position and movements in space by means of the semicircular canals. 



That part of the ear which receives sound waves is known as the cochlea, 

 or snail shell, because of its shape. This very complicated organ is lined with 

 sensory cells provided with cilia. The cavity of the cochlea is filled with a 

 fluid ; this fluid presses against the membrane and separates the inner from 

 the middle car. It is believed that somewhat as a stone thrown into water 

 causes ripples to emanate from the spot where it strikes, so sound waves, 

 transmitted by the bones of the middle ear to the membrane guarding the 

 entrance to the inner ear, are transmitted by means of the fluid filling the 

 cavity to the sensory cells of the cochlea (collectively known as the organ 

 of Corti) and thence to the brain by means of the auditory nerve. 



The Character of Sound. — When vibrations which are received by the 

 ear follow each other at regular intervals, the sound is said to be musical. 

 If the vibrations come irregularly, we call the sound a noise. If the vibra- 

 tions come slowly, the pitch of the sound is low; if they come rapidly, the 

 pitch is high. The ear is able to perceive as low as thirty vibrations per 

 second and as high as almost thirty thousand. The ear can be trained to 

 recognize sounds which are unnoticed in untrained ears. 



Care of the Ear. — Some of the cells lining the cavity of the outer ear 

 secrete a bitter substance called wax. This wax, which aids in keeping the 

 canal of the outer ear moist, also aids in keeping foreign matters, especially 

 living insects, out of the ear. In removing wax or dirt no sharp pointed 

 instrument should be used. Inflammation of the ear should be treated by 

 a doctor, as at such times pus may gather in the inner ear in sufficient 

 quantity to rupture the tympanic membrane. In serious cases, the bone 

 around the inner ear may become diseased and the brain affected. The 

 ear should be protected against sudden loud sounds, such sounds tending 

 to break the tympanic membrane. When the ear is sensitive to cold or 

 dampness, a small wad of cotton may be inserted when going out of doors. 

 This should always be removed upon entering the house. 



The Eye. — The eye or organ of vision is an almost spherical body which 

 fits into a socket of bone, the orhib. What might be the function of this 



