SoO ORGAXS OF SPECIAL SENSE 



stellate figures. The lines observable on the posterior are always so placed as to 

 l)e internifdiale with those on the anterior surface, so that on viewing them through 

 the lens they occupy a position corresponding to the intervals between the lines on 

 the anterior surface. The lens-capsule is comparatively brittle, and can be readily 

 cut through when scraped with a sharjvpointed instrument; on doing so the 

 divided edges curl outwards, away from the lenticular substance. When removed 

 from its capsule, the outer portion of the lens is found to be soft and glutinous, but 

 its substance gets progressively firmer as we approach the centre. This harder 

 central partis known as the nucleus, and the surrounding softer matter as cortex. 

 The cortical part shows a tendency to peel off in successive layers. It consists of 

 long fibres, the ends of which meet in front and behind at the anterior and posterior 

 stellate figures. 



Histologically the capsule is not in immediate contact with the cortex over the front surface 

 of the lens, a single laj-er of cells intervening, called the subcapsular epithelium. 



The suspensory ligament of the lens is formed by a thickening of the ante- 

 rior part of a membrane enclosing the vitreous, strengthened by numerous fibres 

 derived from the folds of the ciliary region. Its chief attachments to the lens-capsule 

 are a little in front of and behind the equator, and the space included between 

 the most anterior and most posterior divisions of the ligament is termed the canal 

 of Petit. This space is bridged across l)y fine intermediate suspensory fibres, and 

 is occupied by fiuid. 



The vitreous humour is a transparent, colourless, jelly-like mass, enclosed in a 

 delicate, clear, structureless membrane, called the hyaloid membrane. This latter 

 is closely applied to the back of the posterior lens-capsule and of the suspensory 

 ligament, and to the inner surface of the pars ciliaris retinae, retina proper, and 

 optic papilla. Although possessing some degree of firmness, the vitreous humour 

 contains quite 98 per cent, of water, and has no definite structure. Membranes 

 have been described in it, but these are really artificial products. In certain situa- 

 tions spaces exist in the vitreous mass, the most determinate of which runs in the 

 form of a canal from the optic papilla to the posterior pole of the lens, corre- 

 sponding to the position of the foetal hyaloid artery (hyaloid canal, or canal of 

 Cloquet). Other very fine spaces are described running circularly in the peri])heral 

 part of the vitreous concentric with its outer surface. Microscopically, wandering 

 cells are found in the vitreous, which often here assume peculiar forms which the 

 observer can, not infrequently, study subjectively. 



The aqueous humour is a clear, Avatery fluid, occupying the space between 

 the cornea on the one hand, and the ciliary body, suspensory ligament, and lens on 

 the other. The iris, projecting into this space, has both its surfaces bathed in the 

 aqueous; but, as its inner part rests on the lens, it is regarded as dividing the 

 space into two parts, an anterior larger, and a posterior smaller aqueous cham- 

 ber, which communicate freely through the pupil. 



Ciliary nerves of the eyeball. — The long and short ciliarj'^ nerves, after per- 

 forating the sclerotic, run forward between it and the choroid to the ciliary region, 

 where they form a plexus, from which proceed branches for the ciliary muscle, the 

 iris, and the cornea. The nerves of the iris enter it at its ciliary border, and run 

 towards its pupillary edge, losing their medullary sheath sooner or later, and 

 supplying specially the s{)hincter muscle. The corneal nerves form an annular 

 plexus near the limbus, from Avhich a few twigs proceed to the sclerotic and con- 

 junctiva, while most of the offsets enter and run radially in the corneal stroma, 

 branching and anastomosing so as to form a plexus. The nerves entering the cornea 

 are about sixty in number, each containing from two to twelve non-medullated 

 nerve-fibres (page 758). 



Blood-vessels of the eyeball. — The ocular tissues receive blood from two sets 

 of vessels, viz. tlie retinal and the ciliary arteries. 



1. The arteria centralis retinae either comes direct from the ophthalmic artery, 

 or from one of its branches near the a]iex of the orhh. Entering the optic nerve 

 twenty millimetres or less behind the globe, it runs forward in its axis to the end of 



