THE ORGANS OF SPECIAL SENSE 26 1 



require adjustment for parallel rays, but for divergent 

 rays a change in the eye is necessary; the ability to make 

 this change is known as the power of accommodation. 



The astigmatic eye is so called from the inability, to 

 focus vertical and horizontal lines at the same ti*ie. This 

 is usually due to some irregularity of the curvpjire of the 

 refracting surfaces of the eye, and a cylindric lens is Re- 

 quired to correct the defect. 



Myopia, or short-sightedness, is a condition caused-jby 

 an increased anteroposterior diameter of the eyeball, 

 causing the parallel rays of light to focus in front of the 

 retina. Myopia causes diminution of distance vision, and 

 requires concave lenses to correct the deficiency. 



Hyperopia,, or far-sightedness, is a condition of refrac- 

 tion of the eye in which, when the accommodation is at 

 rest, the focus of the parallel rays of light Jransmitted 

 through the eye is beyoM the retina. ' Convex glasses are 

 required to correct this defect. 



Presbyopia is a condition of the eye in which the power 

 of accommodation is partly or wholly lost. This condi- 

 tion occurs in the aged, usually commencing between the 

 ages of forty and fifty years, and requires convex spheric 

 lenses for its correction. 



THE ORGAN OF HEARING 



The ear is the organ of hearing, and consists of a series 

 of cavities so arranged as to receive the vibrations (sounds) 

 in the atmosphere, and convey them to the delicate ner- 

 vous membrane within its structure. The ear is composed 

 of three parts : the external ear, the tympanum or middle 

 ear, the labyrinth or internal ear. 



The external ear is composed of two parts the pinna 

 (auricle or pavilion of the ear) and the external auditory 

 meatus (auditory canal). 



The pinna is a cartilaginous plate that surrounds the 



