934 THE SENSORY APPARATUSES. 



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 ihem 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 ciliary processes. This plaited collar has 

 been named the zonula of Zinn {zonula ciliaris, and ora serrata). This zonula — 

 the origin of which has been so much discussed — probably belongs to the vitreous 

 humour. 



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

 retina a small oval elevation, the larger axis of which is about ^ inch ; this little 

 prominence is the optic papilla, or pundum ccecum {papilla conica). From its 

 centre emerge the vessels of the retina. 



At some distance above the pundum cmcum — on the antero-posterior axis of 

 the eye — is the yelloiv spot {macula lutea), in the centre of which is an oval 

 depression — the fovea centralis. This region is the most sensitive part of the retina. 



Structuee. — 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 becomes white and opalescent soon 

 after death. Boll has discovered that, during life, the retina becomes purple 

 in the dark, but regains its normal tint when again exposed to light. This 

 coloration resides in the inner segment of the rod portion. Kiihne has observed 

 that the modification in the retinal purple also occurs when the eye is extirpated. 

 The author has obtained permanent pictures of luminous objects on the retina in 

 treating it with a 5 per cent, solution of alum. The retina-red is regenerated at the 

 expense of the oil drops between the pigmentary layer and the retina. Capranica 

 studied the chemical and spectroscopic character of these drops in 1876-77. 



The retina is composed of connective tissue and nerve-elements, which are 

 arranged to form nine or ten superposed layers. 



Connective Tissue. — This is very delicate and nucleated, and forms two 

 thin layera, 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. 



Nerve-elements. — These are distributed in seven layers, which present 

 the following characters : — 



1. Layer of rods and cones (Fig. 507, 1). — This is also termed the membrana 

 Jacohi {hacillary or columnar layer). It is situated between the inner face of 

 the choroid and the external limitary membrane. 



Rods and cones, regularly mixed, make up its structure. Each of these 

 comprises two portions or segments (separated by a bright transverse line). The 

 outer segment (or shaft) is brilliant and refractive, and consists of a small stalk 

 terminating in a point for the cones ; with a shorter stalk than the inner segment 



