554 THE NERVOUS SYSTEM. 



are absent, and the cones are long and slender. This portion of the 

 retina consists of but little else than cones and the outer nuclear 

 layer ; the cone-fibers are very distinct and nearly horizontal. 



Pars Ciliaris Retinas. At the ora serrata the pars optica 

 retinae terminates, and the pars ciliaris retinae begins. It consists 

 of two layers : An external layer of pigment-cells which are 

 the continuation of the tapetum nigrum, and an internal, of 

 columnar cells with oval nuclei, modified fibers of Miiller. The 

 pigmented epithelium is continuous with the uvea of the iris. 



The sensory epithelium receives its nourishment from the 

 blood-vessels of the choroid. 



Anterior and Posterior Chambers. The anterior chamber is that 

 portion of the cavity of the eye situated between the cornea and 

 the iris ; the posterior chamber, the space between the iris in front ; 

 and the capsule of the lens, the suspensory ligament and the ciliary 

 processes behind. Inasmuch as the iris is in contact with the 

 capsule for the greater part of its extent, this " chamber " is ex- 

 ceedingly small, and hardly deserves the name. In fetal life these 

 two chambers are separated by the membrana pupillaris, but about 

 the seventh or eighth month, when the membrane begins to be 

 absorbed, they become one cavity, and are filled with aqueous 

 humor (p. 556). Some authorities describe the cavity containing 

 the vitreous under the name " posterior chamber." 



Vitreous Body. This is called also vitreous humor. It is a 

 transparent, jelly-like material which fills the cavity of the retina 

 and is enclosed in a delicate membrane, the hyaloid membrane. At 

 the pars ciliaris retinae this membrane splits into two layers, an 

 anterior and a posterior. The anterior layer becomes the suspen- 

 sory ligament of the lens, while the posterior passes behind the 

 lens and covers the anterior portion of the vitreous ; at this part 

 there is a depression in the vitreous in which the lens lies. In the 

 vitreous are found fibers and some cells, the bodies of which con- 

 tain large vacuoles and give off long and varicose processes. 

 Through its center in fetal life runs a small artery from the arteria 

 centralis retinae to the capsule of the lens, but in the adult this is 

 a simple channel, canal of Stilling, which is lined by a portion of 

 the hyaloid membrane. 



Crystalline Lens (Fig. 332). The lens is a transparent body, 

 biconvex in form, the convexity being greater posteriorly than 

 anteriorly. Its transverse diameter is about 0.8 cm., and its 

 anteroposterior diameter about 0.6 cm. It consists of concentric 

 layers or laminae ; those that are centrally situated form the 

 nucleus, and are harder than the external, which are relatively 

 soft. If the lens is boiled or hardened in alcohol, these laminae 

 may be peeled off like the coats of an onion. The laminae are 

 composed of long fibers with serrated edges, which fit into corre- 

 sponding serrations of adjoining fibers. When cut transversely, 



