GENERAL ORGANOLOGY 



121 



the cornea. Internally to the sclera lies the chorioidea, which, at the junction 

 of sclera and cornea, is changed into the iris. Tlie iris, the seat of the color of 

 the e)-e, is pierced by the pupil, which regulates the amount of light. Next 

 internal to the chorioid follows a layer of black cells, the tapelum nignim (pig- 

 mented epithelium), and finally the retina itself, the expansion of the optic 

 nerve which enters the eye at the hinder part. The tapetum nigrum and the 

 retina arise together, and hence both end at the edge of the pupil, although the 

 retina loses its nervous character at the ora serrata, some distance from the outer 

 edge of the iris. 



The cavity of the eye is completely filled by the vitreous body, aqueous 

 humor, and the lens. For vision the lens is the most important, since, next to 

 the cornea, it influences most the course of the rays of light. It lies behind the 

 iris, fixed to the anterior wall of the chorioid, which here is changed into the ciliary 

 process. In front of it is a serous fluid, the aqueous humor, partly in the so-called 

 posterior chamber of the eye, between the lens and iris, partly in the anterior 

 chamber, between the iris and cornea. 

 The single, larger cavity behind the lens 

 is filled up by a jelly-like vitreous body. 

 The image formed on the retina is in- 

 verted. 



Shining of Eyes. — In many verte- 

 brates there is a tapetum lucidum. inside 

 the chorioid which causes the so-called 

 shining of eyes (cats). This is a layer, 

 which reflects light so strongly that only a 

 little light from the outside is necessary to 

 illumine the back of the eye. There is no 

 real production of light. The tapetum 

 nigrum must be free from pigment in 

 order that the tapetum lucidum may act. 

 In many insects and spiders light is sim- 

 ilarly reflected from the back of the eye. 



Phosphorescent Organs. — For a long 

 time eye-like organs have been known, 

 especially in animals from the deep seas 

 (fishes, cephalopods, Crustacea). Many 

 of these have been proved to be organs 

 for the production of light, and the same 

 is probably true of the others. Each is 

 a spherical, eye-like body, arranged in a 

 definite manner in the skin and of very 

 varying structure. Many have a great 

 resemblance to glands (fig. 87). The cells of the gland follicle are apparently 

 to secrete the phosphorescent substance, its light being made more efi'ective 

 by a lens-shaped body of transparent cells and by a reflector (not always 

 present) consisting of strongly iridescent cells, and all surrounded by a 

 pigment layer, the whole being so eye-like that they were at first taken for 

 visual organs. We have in these to do with highly specialized structures 

 differing from the phosphorescent apparatus so common in marine animals of 

 all classes, where {Noctiluca, meduste, corals, etc.), the phosphorescent sub- 

 stance is widely distributed through the body. Perhaps the concentration of 

 the phosphorescence in definite organs may serve to light the surroundings, to 

 attract the prey and perhaps as an attraction between the sexes. In the latter 

 case there would be an analogy with the phosphorescent organs of insects, which 

 are formed in a totally chfferent way. 



Fig. 87. — Phosphorescent organ of 

 Cttauliodus (after Brauer). c, cutis; I, 

 lens; p, pigment layer; s, phosphorescent 

 secretion cells; t, reflecting tapetum. 



