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Note: This text has been automatically extracted via Optical Character Recognition (OCR) software. The text has not been manually corrected and should not be relied on to be an accurate representation of the item.
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branes with which they are lined : also some very minute bones , and the auditory nerve . The external ear , as it is called , consists of two parts , viz . the pinna or ear , popularly so called , and a
tube called the meatus auditorius , which leads from the pinna to the membrana tympani , or from the external to the internal ear . These parts serve for collecting and conveying sounds to the organ of hearing . The passage just mentioned is of
considerable length , and it is lined with a fine membrane , furnished with small hairs for guarding the parts within from the entrance of insects ; and it is moistened by an oily secretion called ceru'inert or wax . The lower part of the external ea ' r , or that which is often
pierced for ear-rings , is called the lobe . The inner extremity of this canal is closed by a thin transparent membrane , which is set in a bony circle , something like a drum-head , hence it has been called the drum of the ear . The internal ear consists of two
divisions , viz * the tympanum and labyrinth . The tympanum is an irregular bony cavity , hollowed out of the temporal bone , just within the membrana tympani , which has several communications with the neighbouring parts .
Opposite to the membrana tympani are two openings thatlead to the labyrinth , which consists ( 1 ) Of an irregular cavity , ( £ ) Of three semi-circular canals which open into this cavity , and lastly , of a spiral canal , something like the shell of a snail , making two turns aud a half from the basis to the
apex , and likewise opening into the former cavity . All these parts of the labyrinth are lined with a fine membrane , which transmits the vibrations it receives from the membrane that separates the labyrinth from the drum of the ear . Besides these parts , there
is the Eustachian tube , that opens iu front of the tympanum . It takes its rise by an expanded cartilaginous orifice at the back of the nostrils , passes through the substance of the temporal bone , and terminates by a contracted orifice in the tympanum .
Its office is to convey air into the cavity of the tympanum . The membrana tympani is thrown into vibrations by the impulse of the sonorous undulations of the air ; but these vibrations « ould not take place unless there were air in the inside as well
Untitled Article
as on the outside of the membranenor could the purpose have beer / answered , had the space been occupied by confined air , because the expansion of that air by heat , or its contract ion by cold , would have distended
or relaxed the covering membrane , in a degree not consistent with the purpose which it was designed to execute . The only expedient , then and that for which the Eustachian tube serves , is to open to this cavity a communication with the external
air : it , in truth , answers the purpose of the hole in a drum . The cavity of the tympanum contains a chain of small bones , called ossicula audituSy the use of which seems to be that of transmitting the vibrations of the air from the membrana
tympani to the labyrinths . These little bones are called ( l ) The malleus or hammer , being larger at one extremity , which is the head , than it is at the other , which is the handle . ( 2 ) The incus , less like an anvil ,
notwithstanding its name , than a grinding-tooth with its two fangs diverging ; in the body of this is a hollow exactly adapted to receive the head of the malleus . ( S ) The stapes , which has an exact resemblance to the iron part of a stirrup . The sense of Hearing , which
requires the assistance of all the parts which have been described , is occasioned by a certain modulation of the air collected by the funnel-like shape of the external ear , and conveyed through the meatus auditorius to the membrana tympani . The air vibratvi
ing on the latter communicates its - brations to the different parts of the labyrinth , and by means of the fluid contained in this cavity , affects a nerve called the auditory nerve , so as to produce sound . Sound is more the
or less loud in proportion to strength of the vibration ; and the variety of sounds seems to depend on the difference of this vibration , w the more quick and frequent it is , tfce more acute will be the sound ,
vice versa . . It should be observed , that tiie filaments of the auditory nerve pass from the meatus auditorius , througft number of very small apertures wiw lead to the labyrinth , and they terminate on the vascular membrane the labyrinth , so that the nerfJJJ pulp is exposed , almost bare , to a
Untitled Article
S 02 Natural Theology . No . F . — Ear *
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Citation
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Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), May 2, 1815, page 302, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct1760/page/38/
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