Nervous System and Sense Organs 223 



The perception of various tones has been best explained by the 

 Helmholts theory. According to this theory, the fibers composing the 

 basilar membrane are of various lengths, varying from the longest at 

 the apex of the cochlea to the shortest at the base. These fibers are 

 thought to vibrate individually, with the shorter fibers being sensi- 

 tive to low tones and the longer ones to high tones. The vibrations 

 set up in these fibers would be similar to those of the strings of a piano 

 or harp. 



SPIBAC LIGAMENT 



Fig. 71. — The organ of Corti. 



While air is the best method of conducting sound waves to the 

 cochlea, vibrations may be also conducted by the bones of the head. 

 This is the basis of some hearing aids. These are, of course, only of 

 use in cases where the auditory nerve or the cochlea are not permanently 

 damaged. If these latter two are damaged, hearing aids may be of 

 no value. 



The Balancing Mechanism. — While the sacculus gives rise to 

 the cochlea, the utriculus gives rise to the semicircular canals which 

 are embedded into the other portion of the bony labyrinth and are 

 filled with endolymph. There are three semicircular canals in each ear, 

 superior, posterior, and horizontal, with each lying at right angles to 

 the other. One pair thus is in each possible plane of movement. At 

 one end of each semicircular canal is an enlarged portion, the ampulla, 

 within which are special columnar, ciliated epithelial cells. At the 

 top of each group of epithelial cells is a gelatinous mass into which 

 the ciha project. Within the utriculus and sacculus are other sensory 

 epithelial cells similar to these. Among these cilia are found a number 

 of small concretions formed of calcium carbonate and known as otoliths. 



