708 HANDBOOK OF PHYSIOLOGY. 



almost disappear, except the rod and cone layer, which considerably in- 

 creases in thickness but at the fovea centralis comes to consist almost 

 entirely of long slender cones and cone-fibres, which curve toward the 

 periphery. They are supported by neuroglia, which is also found inter- 

 nally as a thin layer, the rods being absent. There are capillaries here, 

 but none of the larger branches of the retinal arteries. 



Toward the edge of the macula lutea, not only are all the layers 

 present, but the ganglionic layer consists of many strata of cells (7 or 8), 

 and with this increase there is also an increase in the thickness of the 

 inner granular layer. The cells are generally bipolar. Toward the 

 centre the layers diminish in this order: optic nerve-fibres, ganglionio 

 layer, inner molecular layer, and inner granular layer. The rods grow 

 scanty and then are absent. 



At the ora serrata the layers are not perfect and disappear in this 

 order: nerve-fibres and ganglion cells, then the rods, leaving only the 

 inner limbs of the cones, these cease, then the inner molecular layer. 

 The Miillerian fibres persist. 



At the pars-ciliaris retinae, the retina is represented by a layer of 

 columnar cells, derived from the fusion of the nuclear layers. The cells 

 are covered by the membrana limitans interna, and externally are in 

 contact with the pigment layers of the retina, which is continued over 

 the ciliary processes. 



The chambers of the eye. The anterior chamber is the space behind 

 the cornea and in front of the lens. It is filled with aqueous humor, 

 which is essentially a diluted lymph with a small amount of proteid in 

 it, viz., of fibrinogen, serum-globulin, and septum-albumin. It is seldom 

 spontaneously coagulable. It contains salts, chiefly sodium chloride, 

 sometimes a substance which reduces copper sulphate, but is not sugar, 

 and a trace of urea and sarcolactic acid. There are no formed elements 

 in the fluid. It is stated that the aqueous humor is secreted by glands 

 in the ciliary region, but the cavity is itself obviously a lymph sac. 



The posterior chamber, or that behind the lens, contains the vitreous 

 humor, which is a semifluid substance contained in the meshes of an 

 indistinct connective tissue. It is inclosed in a distinct membrane 

 called membrana hyaloidea, from the anterior surface of this membrane 

 at the ora serrata fibres pass off to the back of the lens capsule, forming 

 an incomplete canal, called the Canal of Petit, the membrane itself being 

 the Zonule of Zinn. The hyaloid membrane separates the vitreous from 

 the retina. 



Blood-vessels of the Eyeball. The eye is very richly supplied 

 with blood-vessels. In addition to the conjunctival vessels which are 

 derived from the palpebral and lachrymal arteries, there are at least two 



