232 ANATOMY FOR NURSES. [Chap. XIX. 



recur at irregular intervals, the only characters perceptible in 

 the sound are its intensity and quality. But if they succeed 

 each other at regular intervals, the sound produced has a 

 position in the musical scale as a high or low note. The more 

 frequent the repetitions, the higher the note ; but a limit is at 

 last reached, at which the ear fails to perceive the sound, and 

 an excessively high note is therefore inaudible. Sonorous 

 vibrations, perceptible to man as musical notes, range between 

 sixteen per second for the lowest notes, and 38,000 for the 

 highest. (Dalton.) 



The sense of equilibrium. — Among the various means (such as 

 sight, touch, muscular sense), whereby we are enabled to main- 

 tain our equilibrium, coordinate our movements, and become 

 aware of our position in space, one of the most important is the 

 action of the vestibule and semicircular canals. The vestibule 

 consists practically of a sac, from the walls of which project 

 sensory hairs, in relation at their bases with the dendrones of 

 the vestibular nerve. Among these hairs rest several small 

 calcareous bodies called otoliths. Each semicircular canal con- 

 sists of a curved tube enlarged at one end (ampulla). In this 



ampulla are hairs around which the 

 dendrones of the vestibular nerve 

 terminate. 



The hairs in the ampullce are stimu- 

 lated by the flowing of the endo- 

 lymph, and the canals are so arranged 

 (Fig. 135) that any movement of the 



Fig. 135. — Diagram show- head causes an increase in the pressure 



ING Relative Position OF THE n ■, -, , . 



Planes in which the Semi- of the eudo-lymph in One ampulla, 



CIRCULAR Canals lie. rl, ^^^^ ^ corresponding diminution in 



anterior vertical canal; P.V., the ampulla of the parallel canal on 



posterior vertical canal; //., ,i -j. • i rni it 



horizontal canal; a, ampulla of ^^^^ opposite Side. Thus a nodduig 



Rt. anterior vertical canal; a', of the head to the right WOUld cause a 



ampulla of Lt. posterior verti- „ pit ir jt-.i 



cai canal. now 01 cudo-lymph irom a to 6 m the 



right anterior vertical canal, but from 

 h' to a' in the left posterior vertical canal. Hence the pressure 

 upon the hairs is decreased in a, but increased in «'. Such 

 stimulations of the sensory hairs are transmitted by the den- 

 drones of the vestibular nerve, through the cell-bodies of the 

 vestibular ganglion and the axones of the auditory nerve, to 



