286 HUMAN PHYSIOLOGY 



the source of the vibrations, e.g. a tuning-fork, is brought 

 into contact with them. 



i . Conduction of sound in the ear to the sensory appa- 

 ratus (see Fig. 38). 



(a) The propagation of sound in the external ear. The 

 external auditory canal (/?, Fig. 38) serves as a funnel which 

 by reflection from its wall gathers the sound vibrations and 

 conducts them undiminished to the ear-drum (cc) which 

 closes the bottom of the canal. The auricle or pinna of the 

 ear is the rudiment of the elongation of this funnel-like 

 passage. The membrana tympani is set in vibration by the 

 vibrations which have been conducted to it. 



(b) The propagation of sound in the middle ear. The 

 middle ear or tympanum (BB, Fig. 38) is a cavity in the 

 petrous bone and contains air. Its outer wall is formed by 

 the drum, its inner wall by a bone in which are two aper- 

 tures closed by membranes, the round and oval fenestrae. 



The membrana tympani is connected with the fenestra 

 ovalis by the auditory ossicles, which convey the vibrations 



of the ear-drum to the mem- 

 brane of the fenestra ovalis. 



Mcp (i'> ?*K!HP r The auditory ossicles are the 



hammer, anvil, and stirrup 

 (stapes), see Fig. 39. 



The manubrium of the 

 hammer, Mm, is united with 

 the ear-drum, lying in its upper 

 vertical radius. From the neck 

 of the hammer proceed two 



Mm ligaments to the walls of the 



FIG. m AUDITORY OSSICLES. , . , .. A . 



,, , r 11 xf j, tympanum which allow the 



Mm, manubrium of malleus; Mcp, ' 



head, and J//, long process, of the ham- hammer to move around on 



mer; fc. incus, or anvil-bone ; fb, short, . , , , , , 



and//, long, process of the anvil; ^, an approximately horizontal 

 stapes. sagittal axis. The head of the 



hammer, Mcp, is united to the anvil by a joint which allows of 

 but little movement, and this movement is largely prevented 

 when the manubrium is moved inward by a coglike process- 



