SENSE ORGANS. x 87 



Johnston, it is also admirably adapted to appreciate 

 both musical tone and direction. If a mosquito be fixed 

 under the microscope and a sound be made on a violin, 

 among the many hairs of all lengths only a few are 

 observed to vibrate in response, viz., those which by 

 length are adapted to co-vibrate. Again, it was observed 

 that in making the sound in different parts of the room, 

 of the hairs pointing in all directions only those vibrated 

 strongly which were at right angles to the direction of 

 the sound. They probably perceive direction much bet- 

 ter than we do.* 



Leaving out, however, these last contrivances as out 

 of the direct line of evolution, and regarding the ves- 

 tibular sac and otoliths as the simplest form of hearing 

 organ, the simplification as we go down the scale is 

 very regular and may in a general way be expressed by 

 the following diagram, which, with the legend, will be 

 readily understood and requires no further explanation : 



Classes 



Outer 



Middle 



Inner Ear 



Conch Meatus 



Mammals 



Birds 



Reptiles 



Amphibians 



Fishes 



Invertebrates 



1 



*~ 



-■4 *--■ 





TT7 

 +-+ — »■ 



TTT" 



IV 

 IV. 



*--*- 



4- 



Fig. 120. — Diagram showing the gradual simplification of the hearing organ 

 as we go down the scale. The stars represent the presence of the parts 

 named above. Middle ear, 1, 2, 3, ossicles ; inner ear, V, SC, C, bony 

 vestibule, semicircular canals, and cochlea ; V, S'C, C", membranous 

 same parts. The dotted continuations of the cochlea line mean that 

 rudiments of cochlea are found in some reptiles. 



* Observe here that the greatest effect is produced when the 

 sound-waves strike the sensitive hairs broadside, and becomes 

 nothing when it strikes end on. In the case of sight the very re- 



