OUR BEST FEATURES 



the middle ear. The stapes was connected with 

 the inner ear on the inner side and by its double 

 outer end (Fig. 112) with both the quadrate bone 

 and the tympanic membrane. When the dentary 

 bone became very large and formed the chief part 

 of the lower jaw, the angular, articular and quad- 

 rate elements, which were still connected with the 

 tympanum, became much smaller. When the 

 dentary formed its new joint with the squamosal 

 (pages 36-39) the lower jaw bones that were 

 behind it (quadrate, articular and angular) gave 

 up their function as jaw elements and intensified 

 their auditory function, transforming sound waves 

 into mechanical pulsations and thus transmitting 

 the equivalents of the sound waves to the stapes; 

 this in turn passed them on to the liquid in the 

 inner ear. 



In this way arose the marvellous delicate mech- 

 anism of the auditory ossicles, the tiny muscles of 

 which (Fig. Ill) are still innervated, even in man, 

 by twigs from the main nerve of the jaw muscles. 

 Meanwhile the first gill pouch, below the back 

 part of the jaw, had grown upward and surrounded 

 the now reduced angular, articular and stapes, 



forming the cavity of the middle ear (Fig. 112). 



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