226 HISTOLOGY 



Its function seems to be the secondary, but important, one of protecting 

 the visual cells or their rods from undue amounts of light as well as form- 

 ing an absorbent background that will free them from unnecessary re- 

 flections. This pigment is sometimes in the visual cells themselves, but 

 more often in other cells that are specialized for this function and found 

 in close connection with the visual cells. It appears as the usual clouds 

 of fine, dark brown granules in the cytoplasm of the cells. 



Some tissue cells in the neighborhood of the visual cells are modified 

 so that their entire body becomes exceedingly transparent and of a high 

 index of refraction. These cells also, either individually or collectively, 

 assume a spherical or other curved form which serves to collect and thus 

 concentrate the power of the rays or even to arrange them as an image 

 on the layer of visual cells. Such a refractive body is the lens of the eye. 

 The lens is not always a cellular structure. Many consist of transparent 

 cell-products, as the cuticular lens of the arthropod eye or the gelatinous 

 lens in the visual organs of some worms. 



In a few rare cases the lens as an image former is done away with 

 entirely, and a diaphragm in front of the retina is used to produce an 

 image, on the principle of the pin-hole camera. This diaphragm also 

 serves to determine the amount of light to be admitted to the retina, and 

 in this capacity it becomes known as an iris. A well-developed iris is 

 found in some eyes that also have a lens, and it serves here to regulate the 

 amount of light that shall enter. It does not take part in the image for- 

 mation when a lens is present. 



When arranged to receive an image, the visual cells, together with 

 some accessory cells, are known as a retina. Many communicatory 

 nerve cells are included in this structure. They sometimes form part 

 of the layer and sometimes are removed from it in ganglia where they 

 have a peculiar arrangement in layers that evidently have some relation 

 to the retina. 



The above structures are all very delicate and, for the most part, must 

 be protected from contact with the exterior. For this purpose we find 

 some cells that make it their duty to form protecting coverings that 

 shield these delicate structures. Such an organ is known as a cornea, 

 and may be composed of a single layer of epithelium, or of an epithelium 

 lying on a connective tissue or of no cells at all, as in the arthropods, where 

 it is the chitinous secretion of certain outer cells. One prominent feature 

 of all cells and tissues that take part in the formation of a cornea or a 

 lens is their transparency. The necessity for this is apparent. 



Visual organs in the Protozoa. The Protozoa that are unques- 

 tionably animals show no clearly defined light- perception region. The 

 general surface of the cell is sensitive to certain intensities of light, the 

 dorsal surface probably being more sensitive than the ventral region. 



