518 APPLIED BIOLOGY 



very near in order to bring it into focus. This is due to 

 the lens being abnormally distant from the retina. Concave 

 glasses should be worn in order to change the direction of 

 rays of light and cause them to focus on the retina. Other 

 eyes are " far-sighted," and only by constant strain can the 

 lens be kept convex enough to focus the rays on the retina, 

 which is abnormally near the lens. Even when straining 

 to the utmost some persons must hold books at arm's length 

 in order to read. Convex glasses correct such difficulty 

 and relieve the excessive strain. In old age the lens loses 

 some of its elasticity and fails to become convex enough 

 when trying to read, and hence a book must be 

 held at a distance, unless convex glasses are used. Astig- 

 matism is a very common congenital or inborn defect due 

 to irregular curvature of the cornea or lens, making it im- 

 possible to see equally well lines which run in different 

 directions, as on a clock-face. For such eyes so-called 

 " cylindrical " glasses should be used constantly to avoid 

 eye-strain. 



443. The Ear. The human organ popularly known by 

 this name is the external ear, from which a tube leads inward to 

 the tympanic membrane, or ear-drum. Beyond this is a cavity 

 known as the middle ear, and from it the Eustachian tube leads 

 to the pharynx. Still deeper in the head than the middle 

 ear is the inner ear, a complicated membranous structure 

 lying in bony cavities of corresponding shape. The lower 

 part (cochlea) is shaped like the cavity in a snail's shell 

 (Fig. 165), and the upper part has three ring-like canals 

 (semi-circular canals). Branches of the auditory nerve con- 

 nect the sensitive membranes of the inner ear with the 

 brain. 



The inner ear is filled with a fluid. Sound vibrations 

 enter the outer tube, and throw the tympanic membrane 

 into vibration. This moves a chain of three bones which 

 extend across the cavity of the middle ear and touch 



