THE SENSES 



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Structure of the Ear. — At one end of the external auditory canal 

 is a piece of membrane stretched completely across it, known as 

 the Tympanum ; it separates the external from the middle ear 

 (Fig. 181). The Middle Ear is on the inner side of the tympanum ; 

 it consists of a cavity containing a chain of very small bones, which 

 stretch like a bridge across the space from the tympanum to the 

 third or internal ear. The middle ear, like the external, is in com- 

 munication with the air by means of a passage known as the 

 Eustachian canal, which opens into the pharynx. The tympanum 

 has, therefore, air on both sides of it, the object of which is to ensure 



Fig. 180. — Diagrammatic Section across the Skull, including Both Petrous 

 Temporal Bones, showing the Relative Position of the Ear to the 

 Guttural Pouches. 



A, Base of cranium ; B. insertion of a muscle of the neck ; C C, petrous temporal 

 bones ; D D, external ear ; D', middle ear ; E E, guttural pouches. The 

 communication of the latter with the pharynx below and the middle ear 

 above has to be imagined (see Fig. 181). The figure also shows, though 

 purely diagrammatically, the position of the semicircular canals and internal 

 ear in the living animal. 



that the atmospheric pressure on either side is equal, and so permit 

 its free swing. The air ordinarily finds its way into the Eustachian 

 tube during the act of swallowing, and by the same channel it is 

 conveyed to the guttural pouches. The Tympanum is concave 

 towards the external ear ; in the middle ear the handle of the malleus 

 is fixed to the central bulging part of it, and as this bone articulates 

 with the incus, and the latter with the stapes, any alteration in the 

 shape of the drumhead, such as is produced by the vibrations of 

 sound, causes the bridge of bones to move ; their movement is 

 further assisted by two small muscles which are attached to them. 



The Internal Ear, known as the labyrinth (Fig. 182), is composed 

 of the semicircular canals, the vestibule, and the cochlea ; these are 



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