ANATOMY. 



na tympani. The situation of the former 

 on the outside of the head is well known; 

 the latter is contained in the petrous por- 

 tion of the temporal bone. 



The external ear consists of two parts, 

 ris. the pinna, or ear, popularly so called, 

 and a tube called ineatus auditorius ex- 

 ternus leading- from the pinna to the mem- 

 brana tympani. These parts serve for col- 

 lect] ng sounds, and conveying- them to 

 the membrana tympani. 



The pinna consists of a convoluted car- 

 tilage, inclosed by common integuments. 

 The lower part, which is pierced for ear- 

 rings, has no cartilage, and is called the 

 lobulus The helix is the fold forming the 

 external circumference of the ear; the 

 next eminence to this, which forms the 

 margin of the great cavity of the external 

 ear, is called anthelix ; it separates at its 

 upper and anterior end into two processes 

 named crura. The projection immediate- 

 ly in front of the meatus is the tragus, and 

 that immediately opposite, the antitragus. 

 The great cavity within the anthelix, and 

 leading to the meatus, is called the con- 

 cha. Several sebaceous glands are situa- 

 ted in the folds of the ear. 



The meatus externus is formed first by 

 a portion of cartilage, continued from the 

 pinna, and more interiorly it consists of a 

 canal in the substance of the bone. This 

 bony part does not exist in the foetus, 

 where the meatus is wholly cartilaginous. 

 The common integuments continued from 

 the pinna line the meatus externus, and 

 the cuticle is produced over the membra- 

 na tympani. 



The surface of the meatus, at its com- 

 mencement, is furnished with numerous 

 fine hairs, and the canal is moistened by a 

 secretion of an oily and inflammable na- 

 ture, called cerumen. This is produced 

 by numerous small glands, visible on the 

 external surface of the meatus, and dis- 

 tingnishahle by their yellowish colour. 

 The cerumen concretes, and is collected 

 sometimes in such quantity as to induce a 

 slight degree of deafness, which is easily 

 removed by svringing with warm wafer. 



The membrana tympani, which ik cir- 

 cular membrane above a quarter of an inch 

 in diameter, is stretched across the inner 

 extremity of the meatus, and derives its 

 name from a comparison with a drum head, 

 to which it bears some analogy in its use. 

 In the foetus it is streiched on a distinct 

 bony ring, called the annulus auditorius. 

 This ring- is deficient at its upper part, and 

 has no bony union to the rest of the tem- 

 poral bone, but it becomes united soon 

 after birth. 



VOL. 1. 



This membrane is concave on its exte- 

 rior surface, and convex towards the tym- 

 panum. Its position is inclined, the upper 

 margin being more towards the outside of 

 the head, and the under part farther in- 

 wards ; so that the superior part of the 

 meatus forms an obtuse angle, and the in- 

 ferior part an acute angle, with the mem- 

 brane. 



The internal ear consists of two divi- 

 sions, viz. the tympanum and the laby. 

 rinth. 



The tympanum is an irregular bony ca- 

 vity, which will about admit the end'of a 

 finger, hollowed out of the temporal bone, 

 just within the membrana tympani. It has 

 several communications with the neigh- 

 bouring parts. 



Opposite to the membrana tympani are 

 two openings, which lead to the labyrinth 

 of the ear. The upper one is named the 

 fenestra ovalis, the lower one the fenestra 

 rotunda, and the projection between them, 

 is called the promontary. The fenestra 

 ovalis is filled, as we shall presently see, 

 by one of the little bones of the tympa- 

 num, and the fenestra rotunda is closed 

 by a membrane. 



The eustachian tube, or iter apalatoad 

 aurem, opens in front of the tympanum. 

 It commences by an expanded cartilagi- 

 nous orifice at the back of the nostrils, 

 passes through the -substance of the tem- 

 poral bone, and terminates by a contract- 

 ed orifice in the tympanum. Its office is 

 to convey air into the cavity of the tympa- 

 num. The membrana tympani is thrown, 

 into vibrations by the impulse of the sono- 

 rous undulations 'of the air, and that vibra- 

 tion could not take place, unless there was 

 air in the inside as well us on the outside 

 of the membrane. Water, or any other 

 fluid, would not have answered the pur- 

 pose. Hence an obstruction of this tube 

 causes deafness, which surgeons have at- 

 attempted to remedy by puncturing the 

 membrana tympani. An opening in the 

 latter membrane of a small extent does by 

 no means injure hearing : for many per- 

 sons have the power of impelling tobacco 

 smoke, or agitating the flame of a candle 

 through the ear, and yet seem to have a 

 perfect use of the organ. In these cases 

 the air or smoke enters the eustachian 

 tube from the throat, and passes through 

 the unnatural aperture in the membrane. 

 The mastoid process of the temporal 

 bone is composed internally of numerous 

 cells, communicating with each other, 

 and finally opening into the back part of 

 the tympanum. These do not exist in the 

 foetus. 



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