THE EAR 169 



ceived by sense organs other than that of hearing, hence a 

 response to a sound is not necessarily an indication of the 

 presence of an organ of hearing. It must also be kept in mind 

 that other animals may make and hear sound vibrations of so 

 high a pitch as to be inaudible to the human ear. 



397. The ear of the grasshopper is called a tympanum, 

 because of its resemblance to a drum head. It is, in fact, a 

 thin membrane stretched over a large respiratory cavity, and 

 is located on the side of the first abdominal segment. The 

 katydid and cricket also have tympanums, but they are located 

 on the tibia of the anterior legs. But in the essential points 

 these organs are similar in structure. The sensory apparatus 

 consists of groups of sensory cells, intimately connected with 

 the inner surface of the tympanum. The tympanum is highly 

 responsive to vibrations of the air, and by its own vibrations 

 the connected sensory cells are stimulated. 



398. At one side of the sacculus, in frogs and reptiles, there 

 is a small pocket which is not found in fishes. In birds this 

 pocket becomes a long tube, and in mammals it is very long 

 and coiled. This is the cochlea and is the true organ of hearing. 

 On one side of the cochlea the lining epithelium is composed of 

 peculiarly arranged columnar cells, which form what is known 

 as the organ of Corti. The cochlea is filled with a fluid, endo- 

 lymph, like the other parts of the labyrinth. In a cross section 

 of the organ of Corti there are several supporting cells and about 

 four sensory cells, but in a longitudinal section there would be 

 from 4,000 to 5,000 sensory cells, covering a space of more than 

 25 mm. The sensory cells are rather stout and rounded at 

 the lower end. At the free end they each bear about twenty 

 rod-like processes, which project into the endolymph. This 

 organ rests on a membrane of fibres (the basilar membrane) 

 which stretches across from the bony wall of one side to that of 

 the other. Above the sensory cells, suspended in the endo- 

 lymph, is a thick cuticular membrane (membrana tectoria), 



