754 THE EYE AND VISION. [CH. LV. 



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 is represented by a layer 

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

 The cells externally are in contact with the pigment layer of 

 the retina, which is continued over the ciliary processes and back 

 of the iris. 



At the entrance of the optic nerve the only structures present 

 are nerve-fibres. 



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

 behind the cornea and in front of the iris. It is filled with 

 aqueous humour, which is diluted lymph. 



The posterior chamber, or that behind the iris, contains the 

 vitreous humour, which is a jelly-like connective tissue (see p. 52). 

 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- 559; P- 74 2 )5 the membrane itself being the Zomde 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 lacrimal arteries, there are 

 at least two other distinct sets of vessels supplying the tunics of 

 the eyeball, (i) The vessels of the sclerotic, choroid, and iris, 

 and (2) the vessels of the retina. 



(i.) 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 very rich choroidal plexus ; 

 they also supply the iris and ciliary processes, forming a very 

 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. 572) are derived from the arteria 

 centralis retinae, 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. 



