148 HUMAN PHYSIOLOGY. 



followed by an oscillation of the head upward and downward ; destruction 

 of the horizontal canal is followed by oscillations from left to right. When 

 the canals are injured on both sides, the animal loses the power of main- 

 taining equilibrium upon making muscular movements. 



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

 appreciating the quality of pitch and the shades of different musical tones. 

 The elements 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 

 a high or low pitch. The vibrations are conducted across the middle ear 

 by the 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 regu- 

 lated 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. 



VOICE AND SPEECH. 



The Larynx is the organ of voice. Speech is a modification of voice, 

 and is produced by the teeth and the muscles of the lips and tongue, co- 

 ordinated in their action by stimuli derived from the cerebrum. 



The Structures entering into the formation of the larynx are mainly 

 the thyroid, cricoid and arytenoid cartilages; they are so situated and 

 united by means of ligaments and muscles as to form a firm cartilaginous 

 box. The larynx is covered externally by fibrous tissue, and lined inter- 

 nally with mucous membrane. 



The Vocal Cords are four ligamentous bands, running antero-posteri- 

 orly across the upper portion of the larynx, and are divided into the two 

 superior or false vocal cords, and the two inferior or true vocal cords ; 

 they are attached anteriorly to the receding angle of the thyroid cartilages 

 and posteriorly to the anterior part of the base of the arytenoid cartilages. 

 The space between the true vocal cords is the rima glottidis. 



