OSSEOUS SYSTEM OF MAMMALIA. 



markable openings : one is termed the Eustachian tube, and it leads to 

 the back of the posterior nares : its commencement is osseous, but it is 

 continued by a cartilage. The use of this tube is twofold : in the first 

 place, it serves, like the hole in the side of a drum, for the necessary pur- 

 pose of establishing a communication between the tympanum and the 

 atmosphere, without which the due vibration of the membrana tympani 

 could not be produced ; in the second place, it assists in the conveyance of 

 atmospheric vibrations to the internal ear : hence persons hard of hearing, 

 as it is expressed, or persons listening very attentively to a distant or 

 slight noise, keep the mouth moderately open. When the Eustachian 

 tube is stopped up, by the inflammation of its lining membrane, from cold 

 or other causes, distinctness of hearing is materially affected. With the 

 tympanum, on the side opposite the Eustachian tube, the cells of the mas- 

 toid process immediately communicate, and may be regarded as an appendix 

 to that cavity. At the extremity of the tympanum are two other orifices, 

 namely, the fenestra ovalis, and the fenestra rotunda (the oval and the 

 round windows), between which the bone is rounded and prominent, form- 

 ing what is termed the promontory. These orifices lead to a maze of 

 cells and tubes, termed the labyrinth : this labyrinth is composed of 

 the vestibule, the cochlea, and the semicircular canals. The vestibule 

 opens posteriorly, by several holes, into the semicircular canals, and by one 

 anteriorly into the cochlea. The semicircular canals are three in number 

 a superior, a horizontal, and a vertical canal. The cochlea, so called from 

 its resemblance to a spiral shell, consists of a tube revolving spirally, to the 

 extent of two turns and a half, round a central pillar, or modiolus. The 

 fenestra ovalis, above referred to, "leads from the tympanum to the vesti- 

 bule, and the fenestra rotunda to the cochlea. Figs. 21 and 22 present 



22 



^*^?*33^ 



21 



Internal ear : a, meatus auditorius ; J>, tympanic membrane; 

 r , the chain of small bones i communicating between the tympa- 

 num and the fenestra ovalis; d, cavity of the tympanum; e, 

 semicircular canals; /, cochlea; K , vestibule, composing the 

 labyrinth ; h, Eustachian tube ; i , auditory nerve, entering at 

 the back of the vestibule. 



An enlarged view of the semicircular canals, 

 vestibule, and cochlea, composing the laby- 

 rinth :e, e, e, the three semicircular canals; 

 /, the cochlea; g-, the vestibule ; fr, the fenestra 

 ovalis ; I, the fenestra rotunda. 



