THE EYE. 827 



elastic layer of the cornea. To examine the structure of the lens, it is best 

 to boil it, or to immerse it in alcohol or 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. According to 

 KoUiker, these fibres are tubular, and contain an albuminous fluid. If any 

 single layer is examined with the microscope, it is found to be made up of 

 these parallel fibres, which measure about ^jTjVtrtli of an inch in thick- 

 ness, and are united with 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 absorb nutriment from the capsule.) 



The lens receives neither vessels nor nerves. In the fcetus, the arteria 

 centralis gives oS a branch which passes forward through the vitreous 

 humour, and enters the posterior face of the lens ; but this vessel disappears 

 long before birth. 



(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 Humour. (Fig. 383, I.) 



The vitreous body, or Jiumour, occupies all the cavity of the eye behind the 

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



It is a kind of colourless, transparent jelly, much more fluid than the 

 lens, and entirely amorphous, according to Ch. Eobin ; though the majority 

 of anatomists add some embryonic cells. At the surface of the vitreous 

 mass is a very thin structure, the hyaloid membrane, which is in contact, 

 externally, with the retina and the posterior face of the lens. 



(This hyaloid membrane forms cells, in which the watery fluid consti- 

 tuting the humour is contained; the cells communicate freely, and are 

 rendered apparent by freezing the eye or steeping it in chromic acid, when 

 it is found that the humour is intersected by a large number of delicate 

 partitions, with a cylindrical space in the axis for the passage of the central 

 artery in the foetus. The membrane is firmer on the surface than else- 

 where, 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 zone of Zinn.) 



There has been described around the lens, between the hyaloid membrane 

 and the zone of Zinn a circular passage named the canal godronne (canal of 

 Petit), because of its form. I look upon this canal as an artificial production, 

 and due to the means employed to demonstrate it in mankind and animals. 



(This humour concurs in refracting the rays of light,) 



3. The Aqueous Humour. 



This is a liquid that owes its name to its great fluidity ; it is contained 

 jn the anterior and posterior chambers of the eye, in front of the lens. It is 

 secreted by a particular membrane, the membrane of the aqueous humour, or 

 membrane of Descemet or Demours ; this is an extremely thin serous layer, 

 easily distinguished on the posterior face of the cornea, and admitted to exist 

 on the two faces of the iris, the ciliary processes, and anterior face of the 

 capsule of the lens, where it is reduced to epithelium only. 



