Page:The American Cyclopædia (1879) Volume VII.djvu/56

 EYE EYLAU the lens dense ; around the entrance of the optic nerve there is a very vascular, horse- shoe-shaped organ, between the layers of the choroid, called the choroid gland or muscle. The organs of vision in insects consist of sim- ple or of compound eyes, the former occurring chiefly in larvee, the latter in perfect insects ; they are wholly absent in some larvae, and both forms coexist in the perfect state of many. The simple eyes (ocelli or stemmata) consist of a convex cornea, behind which is a lens, lodged in an expansion of the optic nerve, and surrounded by a variously colored pigment layer ; they vary in number from two to more than 100, and are situated on the head. The compound eyes are made up of simple eyes so closely placed that their facets or cornesB are contiguous ; behind each cornea is a transpa- rent pyramid whose interior apex is received into a kind of vitreous body, surrounded by the nerve and the choroid; there are sometimes many thousand facets in these eyes, which may cover nearly the whole head, and hairs may project at their angles. In the arachnids the eyes are simple, and the orders have been characterized by their number, situation, and direction ; they are most numerous in the scorpions. The sense of sight is present in almost all Crustacea ; their simple eyes consist of a cornea with a lens and pigment layer ; a usual form is that of many simple eyes, placed close together, and covered by a common cor- nea; sometimes there is a faceted cornea un- der the simple one; the highest forms have compound faceted eyes, in many situated at or near the end of two peduncles movably ar- ticulated to the cephalo-thorax and concealed in special fossae ; these facets are very numer- ous, and behind each is the usual lens and pig- ment. The eyes of cephalopods are very large and highly developed, resembling in some re- spects the vertebrate organ ; there is generally an ocular bulb, and a capsule constituted by a cartilaginous orbit and a fibrous continuation of the cutaneous envelope, which takes the place of a cornea ; semi-lunar folds containing muscular fibres cover the eye like lids ; in front of the globe is a space analogous to an anterior chamber, containing a serous fluid, and in the octopods communicating external- ly ; internally this chamber is closed by a kind of pupil; its serous membrane has a silvery lustre; in. some species the lens is in direct contact with the water in which they swim ; there is an iris, sclerotic, vitreous liquid, a spherical brownish lens formed of concentric layers, a ciliary body, and pigment layer ; in the nautilus the eyes are placed on a project- ing stalk, but in others are generally deeply sunk in the head. In the cephalophora (in- cluding pteropoda, heteropoda, and gasteropo- dous mollusks) eyes are generally present, never more than two in number and compara- tively small ; they are almost always connected with the tentacles, either at their base, sides, or extremities. In acephalous mollusks eyes are very common and numerous, occupying the borders of the mantle or confined to the orifices of the tubes, and are either peduncu- lated or sessile. In the annelids the eyes are generally either wanting entirely, or are mere- ly able to distinguish light from darkness ; but the leeches have 1 from two to ten undoubted eyes. In the helminths there appear to be no eyes, only pigment spots containing no light- refracting body. Below these are found in the radiata various eye specks and pigment dots which doubtless in some cases are true eyes, but authors are not yet agreed as to the light- refracting powers of most of these organs. The eye of the blind fish of the Mammoth cave, Kentucky, though unable to form a dis- tinct image, can doubtless distinguish light from darkness through the areolar tissue and skin which cover it ; Prof. J. "Wyman has found in it a lens, sclerotic, choroid, retina, and optic nerve, and it is therefore constructed on the vertebrate plan, rather than the inverte- brate to which it has generally been compared ; the parts in connection with the nervous sys- tem are developed, while those which are formed by inversion of the integuments are mostly absent ; some authors are of opinion that the stimulus of light for several genera- tions would retransform this eye into an or- dinary organ of vision. EYE STONE, the operculum or calcareous mouthpiece of certain species of small univalve shells. The stony-like substance, one third of an inch or less in its largest dimensions, pre- sents a form like that of a turtle, a convex sur- face upon a plane base ; and being placed on a smooth plate in a weak acid, as lemon juice, the evolution of carbonic acid gas from the carbonate of lime of which it is composed lifts it up and causes the stone to move about as if alive. A similar effect resulting from chemical decomposition is sometimes observed in animal bodies ; and loaves of bread, Hum- boldt remarks, have been observed to move in like manner in the oven, whence the ovens have been called enchanted. He found the little opercula, called piedras de los ojos, or eye stones, regarded as great mysteries by the inhabitants of the coast of Venezuela near Cu- mand. They collected them in great quanti- ties on the beach at Cape Araya, and made use of them to extract dust or any foreign substance from the eye, a purpose for which they are still collected and exported, and are kept by druggists. Being introduced under the lid of the eye, the stone moves about by the motion of the organ, and any little parti- cles it comes in contact with adhere to it and are finally removed with it. EYLAU, or Eilau, a town of Prussia, province of East Prussia, in the district and 22 m. S. S. E. of the city of Konigsberg ; pop. in 1871, 3,V23. It is situated on the Pasmar, a small tributary of the Alle, contains an old castle, and has manufactories of cloth, hats, and leather. Here on Feb. 7 and 8, 1807, was fought a battle