GENERAL OROANOLOGY. 131 



The Eye of the Vertebrates. The eye of the vertebrates 

 usually is an approximately spherical body whose surface is formed 

 by a firm membrane. Over the greater part of the circumference 

 this is an opaque, fibrous or cartilaginous covering, called the 

 sclera, or sclerotica; it is transparent only in the most anterior 

 part, and here it forms by its greater convexity a projecting por- 

 tion like a watch-glass, the cornea. Internally to the sclera lies 

 the choroidea, an envelope of connective tissue, rich in pigment 

 and blood-vessels, which, at the junction of sclera 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 the varying size 

 of which regulates the amount of light. Next internal to the 

 choroid follows a layer of black cells, the tapetum nigrum (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 choroidea, 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 mass of tissue, the vitreous body. The image formed 

 on the retina is inverted. 



The Various Types of Eyes. Between the simple pigment- 

 spot and the highly organized vertebrate eye are many transitional 

 stages: pigment-spots with lens and vitreous body, with enveloping 

 and nourishing coverings, etc. The faceted eye of insects and 

 Crustacea shows a special type of development, described later 

 under the Arthropoda. 



SUMMARY OF THE MOST IMPORTANT POINTS OF ORGANOLOGY. 



1. Organs are composed of tissues, and by their environment 

 are led to the formation of a body of definite shape and to the 

 performance of a single function; consequently every organ is 



