COMPARATIVE. HISTORICAL. 883 



opening in the cornea (sepia, octopus, loligo) ; all other parts of the eyes are well 

 developed. The eye of the vertebrates needs no detailed description. The 

 amphioxus is without eyes, which are undeveloped in proteus, and in the mammal 

 spalax, whose life in the dark has caused the visual organ to atrophy. In many fishes, 

 amphibians and reptiles the eye is covered by skin, which has become transparent. 

 Several varieties of sharks, crocodiles, and birds have eyelids, and, in addition, 

 a nictitating membrane in the inner angle of the eye. Connected with it is the 

 Harderian gland. In mammals the nictitating membrane is reduced to the plica 

 semilunaris. There is no lacrimal apparatus in fishes. The tears of reptiles 

 remain under the watchglass-shaped cuticular covering that extends over the 

 eye. The sclera of the osseous fishes has two bands of cartilage, which are often 

 ossified; from the middle of the choroid, a muscular organ (falciform process) 

 proceeds forward, and its anterior enlarged extremity, which is called the campanula 

 H alien, is inserted into the outer margin of the lens. The campanula, called 

 by Beer the retractor muscle of the lens, pulls the lens nearer to the retina, and 

 in this way produces an accommodation for distance (the eye being accommodated 

 for near vision, when at rest) . In birds the similar muscular structure, the pecten, 

 often reaches nearly to the lens-capsule. The cornea of birds is surrounded by a 



FIG. 312. Eye of a Sea-snail (Patella 

 coerulea), diagrammatic and sim- 

 plified, according to Fraisse; the 

 Nerve according to Hilger: e, 

 body-epithelium; r, retinal cells; 

 n, nerve. 



bony ring. In the birds of prey the cornea 

 changes with the lens. The whale has a tremen- 

 dously thick sclera. The lens in the aquatic 



animals is strongly convex. The muscles of the FIG. 313. Eye of a Sea-snail 



iris and the choroid are striated transversely in tuberculata), diagrammatic and sim- 



reptiles and in birds. It should yet be especially ^SSTSg, %S$S?&S& 



mentioned that the retinal rods of vertebrates body inside; r, retina; n, branched 



(most reptiles have no rods in the retina nerve, 



and no visual purple) are directed from before 



backward, while the analogous elements in invertebrates (crystalline rods, and 

 spheres) are directed from behind forward. In the prehistoric salamanders, 

 the existence of a third eye is assumed in the parietal region (parietal eye) . The 

 pineal gland of vertebrates appears to be the atrophic remnant of the parietal 

 eye. In lizards the parietal eye is present beneath the skin, which is transparent 

 in the iguana, so that it serves here probably in small measure as a visual ap- 

 paratus. 



The investigations of Loeb have shown that (as in plants) the direction of 

 the visual rays has an influence on the direction of movement of many animals 

 heliotropism. In fact many animals without eyes exhibit heliotropism. Some 

 turn toward the light, others away from it. By increasing the temperature or 

 the concentration of the surrounding sea-water, Loeb was able to reverse this 

 action. 



Historical. The Platonics and Stoics considered the visual act as material. 

 Rays of light were supposed to proceed from the eye and from the objects, and 

 to meet, and the rays from the eye to return to it with the feeling of the object 

 The Epicureans believed that small corporeal images proceeded directly from 

 the objects; the Peripatetics that the images were noncorporeal. According 

 to Aristotle the eye does not take from the object any of its substance, but only 

 its semblance, as the wax takes the impression of the seal. The Greeks were 



