156 GENERAL PRINCIPLES OF ZOOLOGY. 



a watch, the cornea. Internally to the sclerotica lies the 

 cJioroidea, an envelope of connective tissue, rich in pigment 

 and blood vessels, which, at the borders of the sclerotica 

 and cornea, is changed into the iris. The iris, the seat of 

 the color of the eye, is pierced in the centre by the pupil, 

 an opening whose varying size regulates the incidence of 

 light. Next internally to the choroid follows a layer of 

 black cells, the tapctnin nignun, and finally the retina 

 itself, the extension of the optic nerve which enters the 

 eye at the hinder part. The tapetum nigrum and the 

 retina are developed together, and hence both end at the 

 edge of the pupil, although the retina loses its nervous 

 character in the ora serrata at some distance from the outer 

 edge of the iris. 



In the interior of the eye is left a cavity, which is com- 

 pletely filled by the vitreous body, aqueous Jiuinor, and the 

 lens. For the act of vision the lens is the most important 

 part, since, next to the cornea, it influences most the 

 course of the rays of light. It lies behind the iris, fixed in 

 the anterior wall of the choroidea, which here is changed 

 into the corpus ciliare. In front of it is a serous liquid, 

 the aqueous humor, partly in the so-called posterior cham- 

 ber 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 

 mass of tissue, the vitreous body. 



The Various Types of Eyes. Between the simple 

 pigment spot and the highly organized vertebrate eye there 

 are many transitional stages of development : pigment-spots 

 with lens and vitreous body, with enveloping and nourish- 

 ing coverings, etc. The faceted eye of insects and crabs 

 shows a special type of development. 



SUMMARY OF THE MOST IMPORTANT POINTS OF 



ORGANOLOGV. 



I. Organs are composed of tissues, and by their envi- 

 ronment are led to the formation of a body of definite 



