36 A TYPICAL VERTEBRATE EYE: THE HUMAN 



image objects as close as one holds a book to read, and must adopt 

 spectacles whether he has ever needed them before or not. This phenom- 

 enon is called presbyopia (literally, old sight), and most of us enter 

 its realm sometime in our forties. The decrease in accommodating 

 power is not caused by any weakening of the ciliary muscle, but by a 

 perfectly normal, progressive hardening of the lens. The ciliary muscle 

 tries as hard as ever in the presbyopic years — but its force, be it remem- 

 bered, is not the one which molds the lens. The actual molding force, 

 the elasticity of the lens capsule, is really quite weak at best, and becomes 

 wholly inadequate to its task when the body of the lens reaches a certain 

 stage of firmness. The hardening of the lens is so gradual, however, that 

 few of us live so long that our graph of the process (Fig. 15) reaches the 

 line of zero accommodation. When this does happen, the once emme- 

 tropic eye is still emmetropic — still focuses parallel rays upon its retina; 

 but its 'near point' (the nearest point at which an object can be sharply 

 imaged) has moved away from the eye until it is twenty feet away, at the 

 point where the eye formerly commenced to accommodate for approach- 

 ing objects. 



(C) The Ocular Adnexa 



The major anatomical structures which fall under the above heading 

 are the oculomotor muscles, the lids, and the lacrimal apparatus. 



The eyeball lies, cushioned by fat, in a pyramidal cavity in the skull, 

 the bony orbit. The angle at the apex of the orbit is about 45°, and the 

 center-lines of the two orbits also make an angle of about 45 . This 

 brings the mesial walls of the orbits approximately parallel; but for the 

 axes of the eyeballs to be parallel it is necessary for them to make 22^/2 

 angles with the axes of the orbits. 



The Oculomotor Muscles — Back at the apex of the orbit is the small 

 aperture by which the optic nerve enters the skull, and close to this 

 point are the origins of four of the six muscles which rotate the eyeball 

 (Fig. 16). These are the straight muscles or 'recti'^ — superior, inferior, 

 medial (internal, nasal) and lateral (external, temporal). They form 

 the 'muscle cone' around the nerve and diverge toward the equator of 

 the eyeball. Here they pass through the connective-tissue capsule (of 

 Tenon) which forms a jacket over the sclera, loosely connected to the 

 episcleral tissue, and which is a portion of an elaborate system of con- 

 nective-tissue membranes or fascia in the orbit, one of whose fortunate 



