806 THE SENSE OF SIGHT 



generally regarded as a sinus-like vein which serves as a drainage tube for the 

 aqueous humor. ^ 



The cornea is not provided with blood-vessels, excepting along its margin, where 

 the conjunctival and sclerotic capillaries form superficial and deep networks. 

 Furthermore, since this structure is also devoid of lymphatics, its nutrition must 

 be effected by the lymph contained in its connective-tissue spaces. It need scarcely 

 be emphasized that this arrangement is of great functional importance, because it 

 enables the rays of light to gain the pupillar aperture without being unduly de- 

 flected from their course. The nerve fibers of the cornea are derived from the 

 plexus annularis surrounding its margin. ^ From here these fibers strive radially 

 into its fibrous substance, where they form secondary plexuses in the anterior 

 and posterior laminated structures. The fibers of these inner networks are 

 non-medullated. 



The choroid is in firm contact with the internal surface of the sclera. It is dark 

 brown in color and extends forward to a point very near the cornea where it 

 terminates in the iris. The latter appears as a transverse fold which is attached 

 to the eyeball at its circumference, but is otherwise freely suspended in the aqueous 

 humor in front of the lens. It will be seen, therefore, that this membranous par- 

 tition divides the anterior cavity of the eyeball into two compartments, called 

 respectively the anterior and posterior chambers of the eye. The former is bounded 

 by the cornea and anterior surfaces of the iris and lens, and the latter by the poste- 

 rior surface of the iris and anterior surface of the lens. Directly behind the 

 iris, the choroid is folded a number of times into a circular thickening which ex- 

 tends into the anterior part of the vitreous humor. This structure is known as the 

 ciliary body and contains the ciliary muscle. Its inner pole gives attachment to 

 the ciliary ligaments which extend from here to the capsule of the lens. The 

 function of these minute parts will be more fully discussed later on when studying 

 the process of accommodation. The choroid consists chiefly of an extensive rami- 

 fication of blood-vessels held in place by delicate strands of connective tissue. 

 These vessels are principally derived from the ophthalmic artery and pierce the 

 sclera externally to the entrance of the optic nerve. They are known as the short 

 posterior ciliary, the long posterior ciliary, and the anterior ciliary arteries. 



The retina, forming the innermost coat of the eye, extends forward to almost 

 the ciliary body. It terminates in this region in a dentated border, known as the 

 ora serrata. Externally, its hexagonal pigmented cells He in close contact with 

 the choroid ; in fact, since these cells most generally remain adherent to the latter, 

 when the retina is peeled off, they are commonly regarded as a constituent of the 

 middle coat. It will be shown later on, however, that they are more intimately 

 related to the retina and should, therefore, be considered as a part of this membrane. 

 The thickness of the retina diminishes gradually from behind forward, measuring 

 0.4 mm. at the yellow spot and 0.1 mm. at the ora serrata. When in a perfectly 

 fresh condition, it exhibits a pink color and appears translucent against the hyaline 

 external investment of the vitreous humor. Its blood-supply is derived from the 

 arteria centralis retinae, a branch of the ophthalmic which enters the eyeball to- 

 gether with the fibers of the optic nerve, and then subdivides in a radial manner 

 until its terminals reach the ora serrata. The microscopic structure of the retina 

 will be more fully discussed later on in connection with its function. 



The Eyelids. — The closure of the eyelids is effected (a) volitionally 

 at irregular intervals, (5) involuntarily at rather regular intervals, 

 (c) reflexly in consequence of the excitation of the trigeminus terminals 

 innervating the structures in the vicinity of the eyeball, {d) reflexly 

 on account of the stimulation of the optic nerve by high intensities of 

 light, and (e) during states of cerebral depression and sleep. The mus- 



^ Dogiel, Anat. Anzeiger, 1890. 



2 Leber and Gidzecker, Archiv fiir Ophthalm., Ixiv, 1906. 



