1018 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



cone-fibers of which are almost horizontal in direction; (3) an exceedingly thin inner plexiform 

 layer. The pigmented layer is thicker and its pigment more pronounced than elsewhere. The 

 color of the macula seems to imbue all the layers except that of the rods and cones; it is of a rich 

 yellow, deepest toward the center of the macula, and does not appear to be due to pigment cells, 

 but simply to a staining of the constituent parts. 



At the ora serrata the nervous layers of the retina end abruptly, and the retina is continued 

 onward as a single layer of columnar cells covered by the pigmented layer. This double layer is 

 known as the pars ciliaris retinae, and can be traced forward from the ciliary processes on to the 

 back of the iris, where it is termed the pars iridica retinae or uvea. 



The arteria centralis retinae (Fig. 879) and its accompanying vein pierce the optic nerve, and 

 enter the bulb of the eye through the porus opticus. The artery immediately bifurcates into 

 an upper and a lower branch, and each of these again divides into a medial or nasal and a lateral 

 or temporal branch, which at first run between the hyaloid membrane and the nervous layer; 

 but they soon enter the latter, and pass forward, dividing dichotomously. From these branches 

 a minute capillary plexus is given off, which does not extend beyond the inner nuclear layer. 

 The macula receives two small branches (superior and inferior macular arteries) from the tem- 

 poral branches and small twigs directly from the central artery; these do not, however, reach 

 as far as the fovea centralis, which has no bloodvessels. The branches of the arteria centralis 

 retinae do not anastomose with each other in other words they are terminal arteries. In the 

 fetus, a small vessel, the arteria hyaloidea, passes forward as a continuation of the arteria centralis 

 retinae through the vitreous humor to the posterior surface of the capsule of the lens. 



The Refracting Media. 



The refracting media are three, viz. : 



Aqueous humor. Vitreous body. Crystalline lens. 



The Aqueous Humor (humor aqueus). The aqueous humor fills the anterior 

 and posterior chambers of the eyeball. It is small in quantity, has an alkaline 

 reaction, and consists mainly of water, less than one-fiftieth of its weight being 

 solid matter, chiefly chloride of sodium. 



The Vitreous Body (corpus mtreum). The vitreous body forms about four- 

 fifths of the bulb of the eye. It fills the concavity of the retina, and is hollowed 

 in front, forming a deep concavity, the hyaloid fossa, for the reception of the lens. 

 It is transparent, of the consistence of thin jelly, and is composed of an albuminous 

 fluid enclosed in a delicate transparent membrane, the hyaloid membrane. It has 

 been supposed, by Hannover, that from its surface numerous thin lamellae are 

 prolonged inward in a radiating manner, forming spaces in which the fluid is con- 

 tained. In the adult, these lamellae cannot.be detected even after careful micro- 

 scopic examination in the fresh state, but in preparations hardened in weak chromic 

 acid it is possible to make out a distinct lamellation at the periphery of the body. 

 In the center of the vitreous body, running from the entrance of the optic nerve 

 to the posterior surface of the lens, is a canal, the hyaloid canal, filled with lymph 

 and lined by a prolongation of the hyaloid membrane. This canal, in the embryonic 

 vitreous body, conveyed the arteria hyaloidea from the central artery of the retina 

 to the back of the lens. The fluid from the vitreous body is nearly pure water; it 

 contains, however, some salts, and a little albumin. 



The hyaloid membrane envelopes the vitreous body. The portion in front of the 

 ora serrata is thickened by the accession of radial fibers and is termed the zonula 

 ciliaris (zonule of Zinri). Here it presents a series of radially arranged furrows, 

 in which the ciliary processes are accommodated and to which they adhere, as is 

 shown by the fact that when they are removed some of their pigment remains 

 attached to the zonula. The zonula ciliaris splits into two layers, one of which 

 is thin and lines the hyaloid fossa; the other is named the suspensory ligament 

 of the lens : it is thicker, and passes over the ciliary body to be attached to the cap- 

 sule of the lens a short distance in front of its equator. Scattered and delicate 

 fibers are also attached to the region of the equator itself. This ligament retains 

 the lens in position, and is relaxed by the contraction of the meridional fibers of 

 the Ciliaris muscle, so that the lens is allowed to become more convex. Behind 



