THE SPECIAL SENSES 133 



Presbyopia is old-sightedness. Here the lens has be- 

 come so hard that the power of accommodation is lost. 

 It usually begins between the fortieth and fiftieth years. 

 Distant vision remains good, but the power of accommo- 

 dation for near vision is lost. 



If the surfaces of the cornea and lens be not segments 

 of a true sphere, there is another error of refraction 

 called astigmatism or inability to see a point. It is 

 often combined with the near- or far-sighted defect. 



The muscles which move the eye ball are supplied by 

 the third, fourth and sixth cranial nerves. The fibers of 

 the ciliary muscle and iris, nonstriated, are. supplied by 

 the ciliary ganglion of the sympathetic .system, which 

 receives motor fibers from the third and sensory from 

 the fifth. There are constrictor and dilator fibers for 

 the pupil, constrictors being stimulated by strong light 

 and dilators by weak. 



HEARING 



The organ of hearing is of such minute and complex 

 anatomic formation that only an outline will be given. 



A very simple, diagrammatic, conception is that of a 

 membrane which vibrates as the result of blows struck 

 by air waves, the membrane being attached to levers 

 which create waves in a fluid which is in contact with 

 the terminals of the auditory nerve which conveys the 

 stimulus to a definite area of the brain. A tense mem- 

 brane vibrates at a given rate only, and responds to 

 high or low rates of vibration as the membrane is 

 stretched or relaxed. A drum is essentially a mem- 

 brane stretched across a circle, and such a membrane, 

 when struck, causes air waves or vibrations at a given 

 number each second; but if the membrane be stretched 

 at one part and relaxed at another, it will practically 



