SYSTEMATIC SYNTHESIS OP TISSUES INTO ORGANS. 313 



and out in the spiral partition between the scalse, and 

 terminate in numerous nerve-cells that are there situated 

 in several rows. 



FIG. 203. 

 D 



N 



Fig. 203, ideal ear. M, the Meatus or external tube ; D, drum, with a 

 chain of bones stretching across it from the external membrane to the in- 

 ternal membrane, covering an opening into the labyrinth, filled with fluid, 

 in which the nerve, N, is suspended. E y Eustachian tube. 



872. THE GENEEAL MODE OF ACTION OP THE EAR 

 MAY BE UNDERSTOOD by observing the preceding figure. 

 The various vibrations of air pass down the tube M and 

 throw its membrane into vibrations ; through the chain 

 of bones the internal membrane is caused to vibrate; the 

 fluid in which the nerve is suspended is thus acted upon, 

 and the nerve consequently affected. 



873. THREE MODIFICATIONS OP THE GENERAL MODE 

 OF ACTION FOR HEARING will be necessary, in accordance 

 with the three classes or characteristics of vibrations, by 

 which they manifest intensity, pitch, and quality. 



874. THE INTENSITY OF VIBRATION is DETERMINED 

 by the space it moves over in a given time ; and that 

 can very well be measured by the action of the vibra- 

 tion on the membrane of the drum, and the muscular 



Describe Fig. 203. What is the difference between nn idal and real Figure ? 

 872. How may ? 873. What ? 874. How is ? 

 14 



