436 Dynamic Theory. 



The comparative anatomy of the eye will be discussed in the next chapter. 

 The human eye is among the best and in some respects is probably 

 the best developed mammal eye. A general description of this eye will 

 serve as a basis of comparison for eyes in general. The eyeball is en- 

 closed on the outside by a tough coat called the sclerotic. The .greater 

 part of it is white and opaque, but in the front part of the ball it becomes 

 transparent, and bulges out with a shorter radius than behind and is 

 therefore more convex. This transparent and more convex part is called 

 the cornea. Inside the sclerotic, and lining it, is a vascular membrane 

 called the choroid. The inner la}^er of this membrane is composed of 

 pigment cells of black color. In the front part of the ball the choroid 

 coat is puckered into about sixty longitudinal ridges which are called 



FIG. 174. Section of Human Eye. 

 C. Cornea. 

 lr.~ Iris. 



Aq. Aqueous Humor. 

 Cm. Ciliary Muscle. 

 C P. Ciliary Process of the Choroid 



Coat. 



C L Crystalline Lens. 

 V Vitreous Humor. 

 K. Recti Muscle. 

 J?. Retina. 



c. Anterior limit of Retina 

 So. Sclerotic Coat. 

 Ch Choroid Coat. 

 Jf/. Yel low Spot. 

 Opt. Optic Nerve. 



the ciliary processes. In front 

 of and lapping over the ciliary 

 processes as if they belonged to 

 an outside layer of the choroid, 

 is a layer of muscular fibres at- 

 tached in the rear to the choroid 

 and in front to the inside of the 

 sclerotic, so that their contraction tends to pull the choroid forward. 

 Attached to the ciliary process is what is called the ciliary zone or sus- 

 pensory ligament of the lens. This is in the shape of a ring, the outer 

 edge being attached to the ciliary processes and the inner edge to the 

 crystalline lens. In shape the lens is convex both in front and rear, 

 but more so in the rear. It does not touch the cornea in front, but is 

 separated from it by a space filled with a transparent albuminous fluid 

 having a little greater density than water and called the aqueous humor. 

 Attached to the sclerotic at the margin where that coat merges into the 

 cornea, and where also the anterior ends of the ciliary process are at- 

 tached, is the outer margin of the circular curtain or diaphragm called 

 the iris. This is colored, and gives the eye its blue, brown, gray, black, 

 &c. , appearance. It has. a hole in the center called the pupil, which ap- 

 pears black on account of the background of black pigment in the 

 choroid coat. The iris automatically contracts or expands in the pres- 



Fio. 174. 



