Integrative Systems 201 



a very thin pigmented epithelium at the base of the retina. Mean- 

 while, a thickened area of the adjacent superficial ectoderm is folded 

 into the interior of the cup, where it acquires the form of a biconvex 

 lens and becomes detached from the superficial ectoderm (Fig. 173). 



These two ectodermal structures, one formed by outgrowth from 

 the brain, the other by ingrowth from the embryonic skin, are sur- 

 rounded by a mass of loosely aggregated mesodermal cells, mesen- 

 chyme, from which are differentiated a nutritive layer, the choroid, 

 adjacent to the retina, and an external supporting layer, the sclera 

 (Fig. 174). 



The sclerotic layer consists of compactly interwoven bundles of 

 connective-tissue fibers in which may develop plates of cartilage or 

 even bone. The layer is opaque except the region of it external to the 

 lens, where it is necessarily highly transparent — the region known as 

 the cornea (Figs. 175, 176). The external skin is closely joined to the 

 outer surface of the cornea but it is exceedingly thin, devoid of pig- 

 ment, and transparent. If eyelids are present, this thin skin, the con- 

 junctiva, continues from the corneal surface over the inner surfaces 

 of the lids (Fig. 176). 



The choroid layer is richly filled with blood-vessels. The main 

 vessels enter the eyeball together with the optic nerve (Fig. 176). The 

 layer is usually heavily pigmented. At the periphery of the cornea, the 

 choroid becomes separated from the inner surface of the sclera and 

 continues as a free membrane, the iris, across the space between the 

 lens and the cornea (Figs. 175, 176). The iris is pierced by an aperture, 

 the pupil, whose center is in the optic axis. The pupil is usually cir- 

 cular, but may be elliptic with the long axis either horizontal or ver- 

 tical. The iris is usually pigmented. It contains two systems of smooth 

 muscle-fibers, radial fibers which dilate the pupil, and circular fibers 

 which contract it. Just beyond the outer border of the iris, the choroid 

 develops a thickened ring of vascular and muscular tissue projecting 

 into the cavity of the eyeball. The ring as a whole is called the ciliary 

 body (Figs. 175, 176). It contains a system of smooth muscle-fibers 

 which extend around the circumference of the ciliary body, and a 

 system of meridional fibers which, as the name implies, lie in meridians 

 of the eyeball, but are entirely in the choroid layer. Their contraction 

 increases the diameter of the ciliary body, thus opposing the action 

 of the circular muscle. Both muscles serve in accommodating the eye 

 for vision at varying distances. 



The lens is a fairly hard but elastic transparent body invested by 

 a highly elastic fibrous outer coat or capsule. The body of the lens 

 consists of cells elongated into thin fibers and compactly arranged in 

 concentric layers. Delicate fibers extend from the periphery of the 



