430 ANATOMY FOR NURSES [Chap. XX 



Vitreous humor. — The posterior four-fifths of the globe of the 

 eyeball is filled with a semi-fluid, gelatinous substance, the vit- 

 reous humor, or body, so called from its glassy and transparent 

 appearance. It is enclosed in a thin membrane — the hyaloid 



membrane. This mem- 



^1 A brane is attached to the 



^>~^r )jf^ retina at the back of the 



\ ^^ /^ eyeball, and furnishes a sus- 



p ic nerM \^k. /jW peusory ligament to the lens. 



Optic chiasm. — ■ \ ^^f^ Elscwhcre it is perfectly 



/ y^^^^^^ separable from its surround- 



optic tract ^y/ ^%^ "^Ss- The vitreous humor 



enclosed in this capsule dis- 



FiG. 203. — DiAGR.wi OF Optic Chiasm. 



tends the greater part oi 

 the sclera, supports the retina, which lies upon its surface, and 

 preserves the spheroidal shape of the eyeball. Its refractive 

 power, though slightly greater than that of the aqueous humor, 

 does not differ much from that of water. 



Crystalline lens. — The crystalline lens is a transparent, refrac- 

 tive body, with convex anterior and posterior surfaces, placed 

 directly behind the pupil, where it is retained in position by the 

 counterbalancing pressure of the aqueous humor and vitreous 

 body, and by its own suspensory ligament described above. 

 The posterior surface is considerably more curved than the an- 

 terior, and the curvature of each varies with the period of life. 

 In infancy, the lens is almost spherical ; in the adult, of medium 

 convexity ; and in the aged, considerably flattened. 



It is a fibrous body, enclosed in an elastic, non-vascular capsule. 

 Just beneath the capsule the substance is soft and gelatinous, 

 but deeper it becomes hard and firm. Its refractive power is 

 much greater than that of the aqueous or vitreous humor. 



Refraction. — Refraction is the bending or deviation in the 

 course of rays of light in passing obliquely from one transparent 

 medium into another of difl'erent density. (See page 496.) 



The refractive apparatus. — In order that our vision of objects 

 looked at should be clear and distinct it is necessary that the rays 

 of light entering the eye should be focussed on the retina. In the 

 normal eye this is secured by the mechanism of accommodation 

 (see next paragraph). The refractive apparatus consists of the 



