118 



PHYSIOLOGY. 



the tipripanum, which in front further communicates very freely 

 with the cavity of the throat through an open canal, the Eustachian 

 tube, whereby air has a free access into the tympanum. This 

 cavity is closed on the outside by the memhrana tympani, which 

 extends over its external orifice as a drum. A small chain of bones 

 extends from this membrane to another in the inner wall of the tym- 

 panum, (the membrane of the foramen ovale.) These are the ossicles 

 of the ear. These small bones are articulated by movable joints, 

 and are moved by small muscles, which are thus enabled to regulate 

 the tension of the membrana tympani, as well as of the membrane 

 of the vestibule. Externally is an apparatus for collecting sounds 

 and conducting them to the tympanum, called the external ear, com- 

 prising the free expanded part, the auricle and the auditory canal, 

 or meatus externum. (Fig. 33.) 



Fig. 33.* 



'" The auditory nerve is the poriio 7nollis of the seventh pair, which 

 is distributed to the vestibule, cochlea, and semicircular canals. Its 

 mode of termination is unsettled. It will be seen from this descrip- 

 tion of the auditory apparatus that it is divided into three parts. 



* 1. Pavilion. 2. Meatus externus. 3. Membrana tympani. 4,5,6. Chain of bones. 7. 

 Cavity of tympanum. 8. Eustachian tube. 9. Meatus internus. 10. Vestibule. 11. Semi- 

 circular canals. 1'2. Cochlea. 13. Stapedius muscle. 



