230 HUMAN PHYSIOLOGY. 



upper extremities are united, forming an arcade. In the internal row it is 

 estimated there are about 3,500 and in the external row about 5,200 of these 

 rods. 



On the inner side of the internal row is a single layer of elongated hair-cells; 

 on the outer surface of the external row are three such layers of hair-cells. 

 Nothing definite is known as to their function. 



The endolymph occupies the interior of the utricle, saccule, and membran- 

 ous canals, and bathes the structures in the interior of the membranous coch- 

 lea throughout its entire extent. 



The auditory nerve at the bottom of the internal auditory meatus divides 

 into 



1. A vestibular branch, which is distributed to the utricle and to the semi- 

 circular canals. 



2. A cochlear branch, which passes into the central axis at its base and as- 

 cends to its apex; as is ascends, fibers are given off, which pass between 

 the plates of the osseous lamina, to be ultimately connected with the organ 

 of Corti. 



Thefunction of the semicircular canals appears to be to assist in maintaining 

 the equilibrium of the body; destruction of the vertical canal is followed by 

 an oscillation of the head upward and downward ; destruction of the horizon- 

 tal canal is followed by oscillations from left to right. When the canals 

 are injured on both sides, the animal loses the power of maintaining 

 equilibrium upon making muscular movements. 



Function of the Cochlea. It is regarded as possessing the power of appreci- 

 ating the quality of pitch and the shades of different musical tones. The ele- 

 ments of the organ of Corti are analogous, in some respects, to a musical 

 instrument, and are supposed, by Helmholtz, to be tuned so as to vibrate in 

 unison with the different tones conveyed to the internal ear. 



Summary. The waves of sound are gathered together by the pinna and 

 external auditory meatus, and conveyed to the membrana tympani. This 

 membrane, made tense or lax by the action of the tensor tympani and laxator 

 tympani muscles, is enabled to receive sound waves of either high or low pitch. 

 The vibrations are conducted across the middle ear by a chain of bones to the 

 foramen ovale, and by the column of air of the tympanum to the foramen 

 rotundum, which is closed by the second membrana tympani, the pressure of 

 the air in the tympanum being regulated by the Eustachian tube. 



The internal ear finally receives the vibrations, which excite vibrations 

 successively in the perilymph, the walls of the membranous labyrinth, the 

 endolymph, and, lastly, the terminal filaments of the auditory nerve, by which 

 they are conveyed to the brain. 



