EYE. 



185 



shews a few shreds of the membrane remaining 

 a week or more after birth. 



Fig. 109. 



Fig. 110. 



The pupil is closed by this membrane during 

 foetal life in order to preserve its dimensions, 

 and secure a correct growth of the iris while the 

 organ is in darkness. If the membrane disap- 

 peared about the seventh month, the pupil 

 should become dilated and remain so during 

 the two succeeding months, unless the muscu- 

 lar power be undeveloped, which is not proba- 

 ble, as it may be seen to operate shortly after 

 birth. 



Of the retina. This is the third spheri- 

 cally disposed membrane entering into the 

 structure of the eye, and may be considered 

 the most essential of all, being that which 

 is endowed with the peculiar description of 

 sensibility which renders the individual con- 

 scious of the presence of light. It is as 

 exactly fitted to the inside of the choroid as 

 that membrane is to the sclerotic, but does 

 not extend to the anterior margin of the choroid 

 as that structure extends to the anterior margin 

 of the sclerotic. The retina is destined to be 

 penetrated by the rays of light, which, reflected 

 from surround ing objects, are collected to form 

 images on the bottom of the eye, consequently 

 its extension as far forward as the choroid or 

 sclerotic is unnecessary, and nature makes no- 

 thing superfluous. It is discontinued at the 

 posterior extremities of the ciliary processes of 

 the choroid, at the distance of about an eighth 

 of an inch from the anterior margin of that 

 membrane. 



The retina is evidently the optic nerve ex- 

 panded in the bottom of the eye in the form of 

 a segment of a sphere. That nerve differs, in 

 some respects, in construction from the other 

 nerves of the body. In its course from the 

 hole in the bone through which it enters the 

 orbit until it enters the eye, it is of a cylindrical 

 form, and proceeds in a waving line to its desti- 

 nation. The medullary fibres are involved in a 

 tough strong material, not separable into cords 

 or bundles as in other nerves, but constituting 

 a cylinder of collected tubes, from the divided 

 extremity of which the medullary matter may be 

 squeezed in as soft and pulpy a form as it exists 



in the brain. It is not easy to determine by 

 anatomical investigation, whether the medullary 

 material is disposed in tubes or in a cellular 

 structure, but as that material is universally 

 disposed in a fibrous form, both in brain and 

 nerve, it is more than probable that it is so ar- 

 ranged here. These cerebral fibres involved 

 thus in a cylindrical bundle of tubes, techni- 

 cally called neurilema by modern anatomists, 

 is covered externally by a fine transparent 

 membrane, adhering to it so closely that it re- 

 quires some care to separate it; and this is 

 again covered by a tube of strong fibrous mem- 

 brane, the sheath of the optic nerve continued 

 from the dura mater to the sclerotic, to which 

 membrane it adheres so firmly, that it cannot 

 be separated except by the knife. Formerly 

 the sclerotic was considered to be a continuation 

 of the dura mater, and much importance, in a 

 pathological point of view, was attached to the 

 circumstance, but although both structures are 

 of the fibrous class, the sclerotic is very different 

 in texture, and the adhesion between them is 

 not more remarkable than any other of the 

 numerous adhesions which occur between fi- 

 brous membranes. 



Where the optic nerve enters the eye, it is 

 contracted in diameter, as if a string had been 

 tied round it, and then passes through a hole 

 in the sclerotic, to which it adheres. When 

 seen from the inside, after removing the retina 

 and choroid, it appears in the form of a circu- 

 lar spot, perforated with small holes, from 

 which the medullary material may be expressed. 

 This is the lamina cribrosa of Albinus, consi- 

 dered to be a part of the sclerotic, but which is 

 really nothing more than the terminating ex- 

 tremity of the nerve. 



The optic nerve does not enter the eye in the 

 centre of the globe, but about an eighth of an 

 inch to the side of it, assuming the centre to 

 correspond to the extremity of a line passing 

 from the middle of the cornea, through the 

 centre of the eyeball to its back. The nerve 

 is generally described and represented as pro- 

 jecting in the form of a round prominence, as 

 it enters the eye ; but this is not, I believe, the 

 state of the part during life, but is produced 

 by the contraction of the neurilema pressing 

 out the medullary matter in this form. As 

 the nerve enters the eye, it immediately expands 

 into and constitutes the retina, the medullary 

 fibres separating and spreading out on the sphe- 

 rical vitreous humour. The expansion of the 

 nerve in separate fibres cannot be distinctly 

 seen in the human eye, but may be recognized 

 with some care in the eye of the ox, and with- 

 out difficulty in that of the hare and rabbit, 

 where it divides into two bundles, as has been 

 well described by Zinn in the Gottingen Com- 

 mentaries. 



The retina does not consist of medullary or 

 cerebral fibrous matter alone. As the brain 

 has its pia mater and arachnoid membrane, 

 and the nerve its neurilema, this nervous struc- 

 ture has its appropriate provision for its sup- 

 port and the distribution of its vessels. This 

 is the vascular layer, first accurately described 

 by Albinus. It is a delicate transparent mem- 



