550 



COLLEGE ZOOLOGY 



eye is nearly spherical. It consists of three 

 concentric coats enclosing transparent sub- 

 stances, as follows. 



1. The outer or sclerotic coat is the white 

 of the eye. It is composed of connective tis- 

 sue and serves as a protective covering. In 

 front of the lens, the sclerotic coat forms 

 a transparent area called the cornea. 



2. The middle or choroid coat is beneath 

 the sclerotic coat; it is supplied with blood 



vessels and contains a great deal of black 

 pigment which prevents light from entering 

 except through the cornea. The choroid coat 

 is separated from the sclerotic coat and is 

 perforated just in front of the lens; the open- 

 ing is the pupil, and a part of the choroid 

 surrounding the pupil is the iris. 



3. The inner coat, the retina, is the most 

 important since it is the light-sensitive layer. 

 It is composed of two types of receptors, 



Sclerotic coat 

 Suspensory ligament 

 Ciliary muscle 



Eyelid 



Choroid coot 

 Retina 



Optic nerve 



Lens accommodated 

 for distant object 



Aqueous 

 humor "- 

 Vitreous 

 humor — 

 Conjunctiva 



Eye muscle 



Blind spot 



Lens accommodated 

 for near object 



Figure 397. Left, the general structure of the eye of man, showing the mechanism of sight. 

 Right, changes in the shape of the lens to focus on distant and near objects. 



rods (about 120 million in a human eye) 

 and cones (about 7 million). The rods are 

 concerned with colorless vision, while the 

 cones are concerned primarily with color 

 vision. The rods contain a pigment known 

 as visual purple (rhodopsin). Light falling 

 on the rods bleaches visual purple, which 

 must be present for vision in dim light. Vis- 

 ual purple is related to carotene compounds, 

 and vitamin A is thought to be a stage in its 

 regeneration; vitamin A deficiency produces 

 "night blindness." At the point where the 

 optic nerve enters the retina, there are no 

 rods or cones. This part of the retina is 



called the blind spot because it is not sensi- 

 tive to light. 



The lens is biconvex and transparent. It 

 is attached to the choroid coat by a suspen- 

 sory ligament. The space anterior to the lens 

 is called the anterior cavity, this in turn is 

 divided into anterior and posterior cham- 

 bers. These two chambers are separated by 

 the iris. The anterior cavity is filled with a 

 fluid called aqueous humor. The vitreous 

 humor, a jellylike substance, occupies the 

 space between the lens and the back of the 

 eyeball. 



The eye is like a camera in certain re- 



