821 TBE APFABATUS OF THE SENSES. 



circumference towards the ciliary ligament to form the dilator of the pupil. 

 The very fine radiating vessels are disseminated among these fibres, and 

 pass to the anterior ciliary trunks. The nerves supplied to the iris are 

 from the ciliary plexus. 



The anterior epithelial layer is composed of the polygonal cells of the 

 aqueous-humour membrane, already described as existing on the posterior 

 surface of the cornea. 



The posterior epithelial layer, or uvea, is constituted by pigment cells 

 analogous to those of the choroid, but less regular in shape. 



In the foetus, the pupil is closed by a very thin transparent membrane — 

 the menibrana pupillaris. (It- is identical with the anterior layer of the 

 capsule of the crystalline lens.) 



5. The Betina. (Pig. 383, d.) 



'Preparation. — Tiie choroid must be removed under water by means of forceps and 

 scissors, after the lens and vitreous humour have been evacuated. A good view of the 

 retina is to be had by looking through the vitreous humour, after the leus and iris have 

 been excised from an eye ) 



The retina, the essential portion of the eye, considered as the terminal 

 expansion of the optic nerve, extends over the internal face of the choroid, 

 from which it is easily separated, and lies between that membrane and the 

 vitreous humour. On arriving at the ciliary body, it is exactly moulded on 

 the radiating folds of its posterior face, and with them is prolonged to the 

 circumference of the crystalline lens, on the capsule of which it appears to 

 become lost, after being closely united to it. It also adheres so firmly to 

 the ciliary processes that, in the fresh eye, it is impossible to detach it. 

 When the eye has been kept some time, however, the two are easily 

 separated; the cornea is removed with a portion of the sclerotic; then 

 dividing the iris into several pieces by diverging incisions, each is turned 

 outwards by a slight traction that ruptures the ciliary zone and the 

 choroid ; the retina being thus divested of the parts which cover it anteriorly, 

 is Seen to form around the lens a kind of Elizabethan ruff, dovetailing with 

 the cUiary processes. This plaited collar has been named the zonula of 

 Zmn (zonula ciliaris, and ora serrata). The majority of anatomists, through 

 having neglected to study this part in fresh eyes, have wrongly considered 

 it as distinct from the retina. 



At the point where the optic nerve enters the eye, there is found on the 

 retina a small oval elevation, whose larger axis is about ^ inch ; this little 

 prominence is the optic papilla or punctum caecum {papilla conica) From 

 Its centre emerge the vessels of the retina. 



Steuctuee.— The retina is the most important of the three tunics of the 

 eye, and it is also the thinnest and most delicate. It forms a soft, pulpy 

 transparent expansion when quite fresh, but is white and opalescent soon 

 alter death. It is composed of connective tissue and nerve elements, 

 which are arranged to form nine or ten superposed layers 



CoNNECTivB Tissue,— This is very delicate and nucleated, and forms 

 two thm lammas named the external and internal limitary membranes; these 

 are connected by radiating fibres which pass through the nerve elements, 

 and anastomose very closely in the molecular layer. 



^EEVE Elements.— These are distributed in seven layers, which present 

 the tollowmg characters : ■/ ^ r 



1. Layer of rods and cones. (Eig. 388, l.)-This is also termed the mem- 



