776 THE EYE AND VISION [oil. LVI. 



It is important to notice what is clearly brought out in fig. 577, 

 that at the fovea, each cone is connected to a separate chain of 

 neurons, whereas in other regions the rods and cones are connected 

 in groups to these chains ; this explains the greater sensitiveness of 

 foveal vision. 



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, next these cease, then the outer 

 molecular layer, the inner and outer nuclear layers coalescing, and 

 finally the inner molecular layer also is unrepresented. 



At the pars-ciliaris retinae, the retina consists of a layer of 

 columnar cells, which probably represent the Miillerian fibres. These 

 cells externally are in contact with the pigment layer of the retina, 

 which is continued over the ciliary processes and back of the iris. 

 Nervous structures are absent. 



At the exit of the optic nerve the only structures present are 

 nerve-fibres. 



The anterior chamber is the space behind the cornea and in front 

 of the iris. It is filled with aqueous humour (dilute lymph). 



The vitreous humour, which is a jelly-like connective tissue (see 

 p. 48), is situated behind the crystalline lens. It is enclosed in a 

 membrane called membrana hyaloidea, which in front is continuous 

 with the capsule of the lens ; round the edge of the lens the canal 

 left is called the Canal of Petit (fig. 561, p. 765), 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 palpe- 

 bral and lacrimal arteries, there are at least two other distinct sets of vessels 

 supplying the tunics of the eyeball. 



(1) These are the short and long posterior ciliary arteries which pierce the 

 sclerotic in the posterior half of the eyeball, and the anterior ciliary which enter 

 near the insertions of the recti. These vessels anastomose and form a rich choroidal 

 plexus ; they also supply the iris and ciliary processes, forming a highly vascular 

 circle round the outer margin of the iris and adjoining portion of the sclerotic. The 

 distinctness of these vessels from those of the conjunctiva is well seen in the 

 difference between the bright red of blood-shot eyes (conjunctival congestion), 

 and the pink zone surrounding the cornea which indicates deep-seated ciliary 

 congestion. 



(2) The retinal vessels (fig. 574) are derived from the arteria centralis 

 retince, which enters the eyeball along the centre of the optic nerve. They ramify 

 all over the retina, in its inner layers. They can be seen by ophthalmoscopic 

 examination. 



The Bye as an Optical Instrument. 



In a photographic camera images of external objects are thrown 

 upon a screen at the back of a box, the interior of which is painted 

 black. In the eye, the camera is represented by the eyeball with its 



