VISUAL TISSUES 22$ 



centers as if it were affected by light waves. It cannot perceive all 

 light waves, and varies considerably in the number of kinds of them that 

 can stimulate it. This cell-organ of sight is known by various names, of 

 which we shall use but two, the rhabdome or the visual rod. 



The rhabdome is placed in various positions on the cell body. It 

 may be on the edge or on the end of the cell and may assume any position 

 of its body to fit in with other optic structures. More rarely it is found 

 inside the cell. It may even be formed upside down in case the cell 

 receives its light rays from behind (eye of man and Pecten). It is not 

 known if the rays of light, striking the plates from their rear or from any 

 other direction than in the front, can stimulate the cell. 



The visual cells are sometimes found scattered on the body surface, 

 but are usually collected into one or two, or even many, groups which, 

 together with the accessory tissues, are called the eyes. Eyes may per- 

 form three functions for their possessor : to perceive the light according 

 to its intensity, or to perceive it according to its direction, or to record 

 light-images of the objects from which the rays come. Some eyes do 

 all of these things. 



To perceive the light alone according to its intensity is a function 

 of the individual visual cell, as well as of the highest eyes. To determine 

 its direction depends upon the position of the visual cell with reference 

 to the body or some larger part of the body. It is also determined by 

 the position of a cell or group of cells stimulated to the exclusion of the 

 rest of the retina. To perceive an image depends upon the relations 

 of many neighboring visual cells to each other and the presence of 

 properly arranged brain centers that can receive their numerous reports 

 as a related whole. 



Some forms of visual cells probably exist that have not been pointed 

 out to science. The surface of the body, in some animals, contains 

 other light-sensitive cells than those in the eyes, and these can perceive 

 light and even the direction of light. The frog forms a concrete exam- 

 ple. If deprived of its eyes, a frog will still be able to orient itself with 

 reference to a ray of light. 



The most primitive form of visual organ would consist pf certain 

 simple epithelial cells, different from their fellows only in their develop- 

 ment of a visual cell-organ or region and their consequent ability to 

 perceive light. An organ of this kind probably exists in such animals 

 as the earthworm, the frog, and some medusae. This brings us once more 

 to realize that very simple eyes will sometimes be met with in highly 

 organized animals. We shall learn later that some simple or lowly 

 organized animals have quite complex eyes, and further, that both kinds 

 of eyes may exist in the same animal. 



Pigment is almost always found in connection with the visual cells. 

 Q 



