796 COMPARATIVE HISTORICAL. 



surrounded by pigment are closely packed together, and are arranged upon a curved surface, so 

 that their free ends also form a part of a sphere. The chitinous investment of the head is 

 facetted and forms a small corneal lens on the free end of each rod. According to one view, 

 each facette, with the lens and the crystal sphere, is a special eye, and just as man has two eyes 

 so insects have several hundred. Each eye sees the picture of the outer world in toto. This 

 view is supported by the following experiment of van Leeuwenhoek : If the cornea be sliced 

 off, each' facette thereof gives a special image of an object. If a cross be made on the mirror of 

 a microscojK?, while a piece of the facetted cornea is placed as an object upon the stage, then we 

 see an image of the cross in each facette of the cornea. Thus for each rod (crystal sphere) there 

 would be a special image. Each corneal facette, however, forms only a part of the image of the 

 outer world, so that we must regard the image as composed like a mosaic. Amongst mollusca, 

 the fixed brachiopoda have two pigment snots near the brain, but only in their larval condition ; 

 while the mussel has, under similar conditions, pigment spots with a refractive body. The 

 adult mussel however, has pigment spots (ocelli) only in the margin of the mantel, but some 

 molluscs have stalked and highly developed eyes. Some of the lower snails have no eyes, some 

 have pigment spots on the head, while the garden snail has stalked eyes provided with a cornea, 

 an optic nerve with retina and pigment, and even a lens and vitreous body. Amongst cepha- 

 lopoda, the nautilus has no cornea or lens, so that the sea-water flows freely into the orbits. 

 Others have a lens and no cornea, while some have an opening in the cornea (Loligo, Sepia, 

 Octopus). All the other parts of the eye are well developed. Amongst vertebrata, Amphioxus 

 has no eyes. They exist in a degenerated condition in Proteus and the mammal Spalax. In 

 many fishes, amphibians, and reptiles, the eye is covered by a piece of transparent skin. 

 [Pineal or Epiphysial Eye. Some lizards, e.g., Hatteria, have a rudimentary median eye in the 

 median line of the head, and lodged in the parietal foramen. It is developed from the pineal 

 body, and its lens is formed from the optic cup, so that light falls upon the retina without 

 penetrating the fibres of the optic nerve. Thus, it is an invertebrate type of eye, where the 

 retina and lens are developed from epidermal structures, while in the vertebrate eye, the retina 

 is developed from the cerebrum.] Some hag-fishes, the crocodile, and birds have eyelids, and a 

 nictitating membrane at the inner angle of the eye. Connected with it is the Harderian 

 gland. In mammals the nictitating membrane is represented only by the plica semilunaris. 

 There is no lachrymal apparatus in fishes. The tears of snakes remain under the watch-glass- 

 like cutis with which the eyes are covered. The sclerotic often contains cartilage which may 

 ossify. A vascular organ, the processus falciformis, passes from the middle of the choroid into 

 the interior of the vitreous body in osseous fishes, its anterior extremity being termed the 

 campanula Halleri. Similarly, there is the pecten in birds, but it is provided with muscular 

 fibres. In birds the cornea is surrounded by a bony ring. The whale has an enormously thick 

 sclerotic. In aquatic animals, the lens is nearly spherical. The muscles of the iris and 

 choroid are transversely striped in birds and reptiles. The retinal rods in all vertebrates are 

 directed from before backwards, while the analogous elements (crystal rods and spheres) in 

 invertebrata are directed from behind forward. 



Historical. The Hippocratic School were acquainted with the optic nerve and lens. Aristotle 

 (384 B.C.) mentions that section of the optic nerve causes blindness he was acquainted with 

 after-images, short and long sight. Herophilus (307 B.C.) discovered the retina, and the ciliary 

 processes received their name in his school. Galen (131-203 a.d.) described the six muscles of 

 the eyeball, the puncta lachrymalia, and tear duct. Aerengar (1521) was aware of the fatty 

 matter at the edge of the eyelids. Stephanus (1545) and Casseri (1609) described the Meibomian 

 glands, which were afterwards redescribed by Meibom (1666). Fallopius described the vitreous 

 membrane and the ciliary ligament. Plater (1583) mentions that the posterior surface of the 

 lens is more curved. Aldrovandi observed the remainder of the pupillary membrane (1599). 

 Observations were made at the time of Vesalius (1540) on the refractive action of the lens. 

 Leonardo da Vinci compared the eye to a camera obscura. Maurolykos compared the action of 

 the lens to that of a lens of glass, but it was Kepler (1611) who first showed the true refractive 

 index of the lens and the formation of the retinal image, but he thought that during accommo- 

 dation the retina moved forward and backward. The Jesuit, Scheiner (+ 1650), mentions, how- 

 ever, that the lens becomes more convex by the ciliary processes, and he assumed the existence 

 of muscular fibres in the uvea. He referred long and short sight to the curvature of the lens, 

 and he first showed the retinal image in an excised eye. With regard to the use of spectacles 

 there is a reference in Pliny. It is said that at the beginning of the 14th century the Floren- 

 tine, Salvino d'Armato degli Armati di Fir (+1317), and the monk, Alessandro de Spina (+1313), 

 invented spectacles. Kepler (1611) and Descartes (1637) described their action. Mayo (+1852) 

 described the 3rd nerve as the constrictor nerve of the pupil. Zinn contributed considerably 

 to our knowledge of the structure of the eye. Ruysch described muscular fibres in the iris, and 

 Monro described the sphincter of the pupil (1794). Jacob described the bacillary layer of the 

 retina Soemmering (1791) the yellow spot. Brewster and Chossat (1819) tested the refractive 

 indices of the optical media. Purkinje (1819) studied subjective vision. 



