AUDITORY TISSUES 22 3 



We have here ample proof that the hair cells, both outer and inner, 

 are the sensory auditory cells. The stiff, short rods set in their upper 

 surfaces are the cell-organs of auditory perception, and receive the 

 stimulus. 



Our next concern is to know as well as we can how this stimulus 

 is imparted to the auditory rods. Do the waves of sound vibrate them, 

 or is there an intermediate tissue or substance ? 



As in the tactile and static tissues, we find that here an intermediate 

 substance is present and is probably necessary. This is a plate or shelf 

 of material which projects from the elevation on the left, the labium 

 vestibularis, and reaches across to cover the hair cells with its edge. It 

 is called the membrana tectoria and is probably a cuticular product of the 

 labial cells. Such a broad surface must easily be made to vibrate by the 

 waves of sound, especially as they come intensified by accessory tissues, 

 and must thus play mechanically upon the auditory rods and give them 

 a characteristic stimulus. 



The accessory tissues are the pinna, a shell-shaped organ designed to 

 catch a volume of sound waves, and, having concentrated them, to pro- 

 ject them through a tube and against a stretched membrane, the tym- 

 panum. In a frog this tympanum is larger and directly exposed to the 

 air waves without the aid of such accessory tissues. 



The tympanum is composed of both connective-tissue and epithelial 

 elements, and the four layers are, from without inward, a thin stratified 

 epithelium called the stratum cutaneum; a layer of connective-tissue 

 fibrils arranged as a radiating tendon and known as the stratum radia- 

 tum ; another connective-tissue layer called the stratum circulare ; and an 

 internal layer of simple cuboidal epithelium that is continuous with the 

 epithelium lining the middle ear. The three tiny bones which form a 

 chain to transmit the vibration to the internal ear-sac are composed of 

 a very fine and dense bone tissue. 



Technic. The same remarks as to technic may be applied here as 

 were found following the previous part. It may be added that no care- 

 ful studies of a tissue of hearing can be satisfactory if they are not 

 grounded on experimental work that demonstrates the tissues to be 

 actually sensitive to sound stimuli. The auditory tissues of insects are 

 very hard to handle by the section method, owing to the chitinous struc- 

 tures with which they are associated. When the insects are small and 

 have a delicate cuticle, the sections may be easily secured. Also when a 

 larger insect with a heavy cuticle has just emerged from the molt, its 

 shell is soft and may be ignored. Chitin can be softened, but always 

 at the expense of the other tissues. It is better to remove the chitin if 

 possible. 



