TEE MEDIA OF THE EYE. 937 



prismatic bands (the fibres of the lens, themselves extremely long epithelial cells) 

 denticulated on their borders, and provided with one or more nuclei. 



The epithelium on the inner face of the capsule becomes dissolved soon after 

 death, and forms the liquor Morgagni^ which is, consequently, nothing more 

 than a, post-mortem product. 



The lens has neither vessels nor nerves. 



In the foetus, it receives the central artery of the retina— a branch which 

 passes forwards through the vitreous humour and enters the posterior face of the 

 lens ; but this vessel disappears a long time before birth. 



(It should be noted that the fibres of the lens pass in a meridional direction, 

 but none go beyond the entire half of the lens — as the nearer a fibre starts 

 from the anterior pole, the further it is removed from the posterior pole at its 

 termination. 



To examine the structure of the lens, it is best to boil it, or to immerse it in 

 alcohol ,or very diluted nitric acid, which renders it hard and opaque. It is then 

 found to be divided into three equal parts by three lines, which radiate from the 

 centre to within one-third of the circumference ; so that each of these portions 

 is composed of hundreds of concentric layers, arranged within one another, like 

 the coats of an onion. If any single layer is examined with the microscope, it 

 is found to be made up of these parallel fibres, which measure about :5^oW of 

 an inch in thickness, and are united to each other by finely serrated or scalloped 

 borders that dovetail in the most beautiful manner. 



The lens is nourished by means of the extremely delicate layer of nucleated 

 cells on its surface, which absort nutriment from the capsule. 



The use of the lens is to bring the rays of light to a focus upon the retina, 

 they being greatly refracted in passing through it.) 



2. The Vitreous Humoue (Fig. 502, 1). 



The vitreous hody, or humour, occupies all the cavity of the eye behind the 

 lens (about two-thirds of the interior of the eye). 



It appears to be a colourless, transparent jelly, much more fluid than the 

 lens, and is formed of a fluid amorphous substance contained between layers of 

 extremely delicate connective tissue, anastomosing with each other, in every 

 direction. On the surface these layers join a thicker one, contiguous to a thinner 

 that envelops the whole mass, and named the hyaloid membrane. Beneath this 

 membrane, in front, are bundles of connective tissue which gradually diverge 

 widely forward, and become attached to the front and posterior circumference 

 of the capsule of the lens, to constitute the zonula of Zinn, 



(This humour also contains round cells like leucocytes, and stellate and 

 spindle-shaped cells, sometimes showing vacuoles in their protoplasm. The 

 spaces in the humour communicate freely, and are rendered apparent by freezing 

 the eye or steeping it in chromic acid, when it is found that the humour is inter- 

 sected by a large number of delicate partitions, with a cylindrical space — canalis 

 hyaloideus, or the canal of Cloquet — in the axis, for the passage of the central 

 artery in the foetus. The menibrane is firmer on the surface than elsewhere, so 

 that it serves as a capsule for the humour, and suffices to keep it in shape after 

 the outer envelopes of the eye are removed. As mentioned, the lens is maintained 

 in situ by the zonula of Zinn. 



This humour concurs in refracting the rays of light.) 



