ANATOMY. 



to the selerotica by aloose adhesion, which 

 ran be destroyed by blowing air between 

 the membranes; but in front this adhesion 

 is stronger, and forms a white circle named 

 orbiculus, or ligamentum ciliare. The co- 

 lour of the choroid cout is a deep brown, 

 approaching to a black, and this colour is 

 derived from a substance called pigmen- 

 tum nigrum, which separates from the 

 choroid by maceration, and dissolves in 

 water so as to render it turbid. 



The inner surface of the choroid coat, 

 which is universally coloured by pigmen- 

 tum nigrum in the human subject, is 

 sometimes called tunica ruyschiana, as 

 Ruysch endeavoured to prove that it form- 

 ed a distinct membrane from the external 

 part. It is this inner surface that possess- 

 es the brilliant colours observable in ani- 

 mals, whence the appellation of tapetum. 

 This surface lies in contact with the retina, 

 but does not adhere to that membrane. 

 On the front of the eye, however, and be- 

 yond the anterior margin of the retina, the 

 choroid is closely attached by means of 

 numerous and very delicate folds, called 

 the ciliary processes, to the surface of the 

 vitreous humour, round the margin of the 

 crystalline lens. 



The iris is a membrane continued trans- 

 versely across the eye-ball, behind the cor- 

 nea, and appearing as a continuation of 

 the choroid from the orbiculus ciliaris. 

 The round opening in the front of this 

 membrane is called the pupil ; it allows 

 the passage of the rays of light into the 

 interior of the eye. This aperture varies 

 in its dimensions according to the quantity 

 of light to which the organ is exposed : a 

 strong light causes the pupil to become 

 contracted, in order to exclude a portion of 

 the rays of light which offend the organ. 

 The aperture is dilated in a weak light, to 

 let in as many rays as possible. Some 

 anatomists have thought proper to employ 

 themselves in debating at length, whether 

 these motions arise from a really muscular 

 structure or no ; but we believe that they 

 iiave not yet settled the point completely. 



The name of iris 'was applied to this 

 part, from the diversity of colours observ- 

 able in it in different individuals ; and 

 it is the colour of this that produces the 

 colour of the eye, in the popular sense of 

 'he phrase. There is a remarkable cor- 

 respondence in this point between the 

 skin and hair and the iris. A light com- 

 plexion and hair is accompanied with blue, 

 grey, or the lighter colours of the iris; 

 while a dark skin and black hair are at- 

 tended with the dark brown iris. 



In that curious variety of the human 

 race called the Albinos, where the skin 



and hair are of a dead milk-white hue, if* 

 consequence of a total absence of the 

 rete mucosum, or colouring principle, the 

 colouring matter of the iris and choroid is 

 also deficient, and these parts appear red, 

 from the numerous blood-vessels which 

 they contain. 



The posterior surface of the iris is co- 

 vered by pigmentum nigrum, and is call- 

 ed the uvea. 



Under the choroid coat is found a third 

 membrane of the eye-ball, called the reti- 

 na, which is formed by the expansion of" 

 the medullary substance of the optic nerve, 

 and forms the immediate organ of vision, 

 It is of a yellowish grey colour, and so ex- 

 tremely soft as almost to be lacerated by 

 the slightest touch. Its outer surface is 

 entirely unconnected with the choroid 

 coat ; and the inner surface is expanded 

 on the vitreous humour, but not connected 

 to it. It terminates in front by a distinctly 

 defined edge, where the ciliary processed 

 begin to adhere to the vitreous humour. 

 On the inside of the retina are seen the 

 branches of an artery and vein, which en- 

 ter through the centre of the optic nerve, 

 (arteria et vena centralis oculi). The part 

 at which it enters the eye is termed the 

 porus opticus, and is of course insensible: 

 and hence physiologists have explained 

 the reason why the optic nerve is inserted 

 out of the axis of the eye ; as otherwise 

 the axis of vision would have fallen on an 

 insensible part of the retina. 



On the outer, or temporal side of the 

 retina, there is a fold of the membrane of 

 a bright yellow colour, in the recent state, 

 and there is also said to ;,be an aperture. 

 These circumstances were first pointed 

 out by Soemmering, and have been 

 named after him. 



The vitreous humour occupies the great- 

 est share of the globe of the eye. It con- 

 sists of a clear water contained in a cellular 

 substance, which is so perfectly transpa- 

 rent as to resemble pure glass, whence its 

 name is derived. The cellular substance 

 is condensed on the surface into a smooth 

 membrane, called the membrana hyaloi- 

 dea. This is marked in front by a circular 

 series of black radiated lines, caused by 

 the adhesion of the ciliary processes, 

 which, like other parts of ihe choroid, are 

 covered with pigmentum nigrum. Under 

 these a circular canal runs, named the ca- 

 nal of Petit. 



The crystalline humour or lens is im- 

 bedded in the front of the vitreous hu- 

 mour. Its size is about that of a pea, but 

 it is much more flattened in form. Tt is of 

 a waxy consistence, softer external!}', and 

 growing gradually firmer towards the cen- 



