360 



THE MAINTENANCE OF THE INDIVIDUAL 



l)osterior part of the retina lies a region known as tlie yellow spot, or 

 macula lutea. The central pitlike portion of the macula lutea, where 

 cones predominate, almost to the exclusion of rods, is designated as 

 the fovea centralis, since it is here that the keenest vision occurs. 

 The retina is thinner at this point and the black pigment of the outer 

 layer shows through from behind, making it dark purple in color, due 

 to a layer of cells next to the choroid coat. The retina acts as the 



sensitized plate in a camera, for 



outar Surface of retivict 



*- pigment 

 layer 



....l-oct 



...Cone- 



outar 



nuclear 



layer 



(outer , 

 I crarjular 

 *^layar 



inner- 

 groiT7ixlaTr 

 layer 



1 ganglionic. 



jlell^layer 

 fibens- of 

 optic nerve 



on it are received the impressions 

 of light and shade and color which 

 are transformed and sent to the 

 brain resulting in sensations of 

 sight. Like the camera, the eye 

 has a lens formed of transparent 

 elastic material, a circumstance 

 permitting a change of its form 

 and, in consequence, a change of 

 focus upon the retina. By means 

 of this change in form, or accom- 

 modation, both near and distant 

 objects may be seen. In fishes, 

 unlike mammals, accommodation 

 is accomplished by shifting the 

 position of the lens, as in a camera, 

 rather than by changing its shape. 

 In front of the lens is a small 

 cavity, divided by the iris into 



inner surface of" retina. 

 Detail of retina showing rods and cones. 



two chambers that communicate through the pupil, filled with a 

 watery fluid, the aqueous humor, while behind it is the main cavity 

 of the eye, filled with a transparent, almost jellylike, vitreous humor. 

 The lens lies directly behind the iris and is attached to the choroid 

 coat by means of delicate ligaments and by pressure of the two 

 liquid media. 



In order to function properly, the surface of the eye must be kept 

 moist, and various glands are located in the cavernous orbit of the eye 

 and along the edges of the eyelids which serve this purpose. The best 

 known are the tear or lachrimal glands with their associated ducts that 

 open into the nasal chamber. These glands increase their normal 

 production of moisture to form visible tears when the surface of the eye 

 is irritated by foreign particles or when the emotions gain control. 



