THE NERVOUS SYSTEM 193 



the base of a narrow cone-shaped structure, the ommatidium, 

 whose inwardly directed apex rests against a hemispherical 

 basement membrane. Each ommatidium contains a pear- 

 shaped lens supported by cells beneath the cornea. The remain- 

 der of the ommatidium is usually made up of seven elongated 

 retinal cells, extending from the lens to the basement membrane. 

 The surface of each retinal cell facing the cavity of the ommatid- 

 ium is provided with rows of transverse plates or rods, which 

 constitute a rhabdome. Since the plates or rods of the rhabdome 

 project at right angles to the axis of each retinal cell, they are in a 

 position to intercept light entering the ommatidium through the 

 cornea. The lower or inner ends of the retinal cells are pro- 

 longed into nerve fibers to form an optic nerve leading to the 

 central nervous system. Pigment cells occur between the cornea 

 and the outer edges of the lens and also about the retinal cells so 

 that the amount of light entering the ommatidium may be 

 controlled (Fig. 126). 



The compound eye is provided with all the elements necessary 

 to produce an image of a mosaic type, each ommatidium in the 

 proper location with reference to the object forming a portion of 

 the total image. The number of ommatidia affected depends 

 upon the degree to which the ommatidia are sheathed by pigment 

 cells. When the pigment is extended about the ommatidia, 

 only those ommatidia will be stimulated whose axes are in line 

 with the source of light. When the pigment is withdrawn a 

 larger number of ommatidia will be affected by a single source of 

 light. Such an eye would seem to be especially adapted for 

 recording a moving object, since a change in position of the 

 object would affect in turn a larger number of ommatidia than 

 would be the case if the object were stationary. 



Human Eye. — The eyes of vertebrates are definitely image- 

 forming organs, conforming to a common structural plan, though 

 differing in details. The general plan of structure can be illus- 

 trated by the human eye. The shape of the human eye is roughly 

 that of a sphere whose curvature in the region of the transparent 

 part in front is somewhat greater than elsewhere (Fig. 127). 

 The outer layer of the eye is made up of the transparent cornea 

 in front and the tough, opaque sclera or sclerotic coat, forming 

 the remainder. The visible portion of the sclera is the white of 

 the eye. Beneath the sclera lies the vascular choroid layer, 



