Bt SENSE OF HEARING. 



sounds ; for it is so arranged, that it can be stretched or relaxed ; 

 and when stretched, it transmits sounds less perfectly. 

 Fiy. 29. 44. We also remark in the interior of the tympanum, 

 m a transverse chain, formed of four small bones, named 

 C\ on account of their shape, (Fiy. 29.) the malleus, (m.) 

 -^F (hammer,) incus, (en.) (anvil) lenticular bone, (I.) or os 

 * orbicular?, and stapes, (*/.) (stirrup.) 



45. The malleus rests upon the membrane of the 

 tympanum, and affords attachment to muscles, which, 

 by contracting, may cause it to press more or less strongly 

 upon the membrane ; in this way it is stretched or re- 

 laxed to adapt itself to the intensity of the sounds by 

 O l which it is struck. 



/gv st 46. In the interior of the cavity of the tympanum, 

 ^"^ there are two small openings which are closed up by 

 membranes stretched over them p. 

 like that of the tympanum ; they ^' 



lead to the internal ear. One of 

 them, called the feneslta oralix, or 

 foramen ovale, is in contact with 

 the base of the stapes; the other, m 

 called the fenestra rotunda, or foi- 



amen rotundvm, is situated a little '""", ''f^HHF et 



lower down. The cavity of the 

 tympanum also communicates with a great number of cells which 

 are in the substance of the petrous bone. 



47. The internal ear is composed of three parts, namely, the 

 vestibule, the semi-circular canals, and the cochlea, (Fiy. 28.) 

 These organs are tilled with a watery liquid, in which the fila 

 merits of the acoustic nerve terminate. 



48. The vestibule, and the acoustic nerves constitute the 

 essential part of the auditory apparatus ; the other parts which 

 we have just enumerated are destined to perfect this apparatus, 

 and for the most part, may be destroyed, even in man, without 



Explanation of Fig. 30. The tympanum with the bones of the ear,- L 

 the tympanum, ma. the maleus or hammer, m, the handle of the maleus, 

 which rests upon the tympanum, mm. muscles of the maleus, en. the 

 incus, 01 anvil, et. the stapes or stirrup, me. muscles of the stapes. 



44. What is found in the interior of the tympanum ? 



45. What bone gives attachment to the small muscles which act on the 

 membrane of the tympanum / 



46. In the cavity of the tympanum are found two small openings ; to 

 wl.at part do they lead * VVhich foramen has the stapes bone attached to 

 it? With what other parts does the cavity of the tympanum communicate? 



47. Of what parts is the internal ear composed? With what is thes 

 organs filled, and what terminates in them ? 



48. What essentially constitutes the auditory apparatus? 



