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The layer of ctneritious cerebral substance is pretty sharply 

 <lrtine.i on the fide of the granular layer, and less so towanU that of 

 the fibre* of the optic nerve, between the clement* of which it 

 penetrate* more or lew. It is composed of n finely granular matrix, 

 corresponding exactly with that of the gray substance on the surface 

 of the cerebrum and cerebellum, and of numerous nerve-cell* 

 scattered iu it 



The expansion of the optic nerve. This nerve after quitting the 

 brain and till it reaches the eye present* the same conditions as in 

 the ordinary nerves. Within the cannl of the sclerotic and as far 

 as the colUculus nervi optici, the optic nerve retains its white 

 colour, and present* dark-bordered tubules, but from that point 

 onwards its elements become perfectly clear, yellowish or grayish, and 

 transparent, like the finest tubules in the central organs, measuring on 

 the average not more than 0-0006'" 0-0008'" ; some it is true lire 

 occasionally larger. What chiefly distinguishes these from other 

 pale nerve-terminations, is the absence of nuclei in their course, a 

 somewhat greater refractive power, and the frequent occurrence of 

 varicositiee, which two latter particulars would seem to indicate, if 

 not exactly a nerve-medulla, as in the common nerves, still the 

 existence of partially semi-fluid and perhaps fatty contents, and 

 animilate the nerve-fibres of the retina to the most delicate elements 

 of the cerebrum. The nerve-fibres radiate on all sides from the 

 colliculus nervi optici, and constitute a continuous membranous 

 expansion, which extends as far as the ora serrate retime, and presents 

 very considerable interruption only in the situation of the macula lutea, 



The limitary membrane is a delicate membrane, 0-0005'" thick, 

 intimately united with the rest of the retina, which when that 

 structure is teased out, and on the application, is frequently detached 

 in large threads, and then appears perfectly structureless. On its 

 inner aspect towards the hyaloid membrane, when the retina is folded, 

 flattened cell-nuclei are occasionally perceptible, which certainly 

 cannot be referred to an epithelium, and Kolliker thinks not to the 

 vitreous lens either, as the latter is always separable from the retina. 



The nature of the rods and cones in the bocillar layer have been 

 much discussed. It has been supposed that they are the terminations 

 of the optic fibres. There are however difficulties in the way of this 

 theory ; " among which," says Kolliker, " not the least is the circum- 

 stance that although the ' rods ' and ' cones ' are certainly fifty times 

 more numerous than the fibres of the optic nerve, yet the radiating 

 fibres arising from the former, on their passage into the optic fibres, 

 subdivide, and as it must probably be assumed, are continuous with 

 several of them." " I look upon the ' rods ' and ' cones,'" he continues, 

 " which may also be said to correspond in all chemical characters 

 with the nerve-fibres of the retina, aud the whole of the radiating 

 fibre-system of the retina, as true nervous elements ; and venture at 

 the same time to broach the bold supposition, founded upon a less 

 established basis, that the ' rods ' and ' cones ' are the true percipients 

 of light, and that they communicate their condition to the fibres 

 of the optic nerve, by means of the direct or indirect connection of 

 their fibrous processes with the former, through which again the 

 impressions are conveyed to the sensorium." He think.* that the 

 optic fibres in the nervous expansion of the retina do not perceive 

 light, for the following reasons : 1. That the point of the retina, 

 where those fibres alone are found, is not sensible to light. 2. The 

 optic fibres are superimposed upon each other in such numbers 

 that it is impossible they should perceive light, inasmuch as each 

 luminous impression, owing to the transparency of the fibres, must 

 in any case always affect many of them, and consequently would of 

 necessity give rise to confused sensations. 3. Because this part of the 

 retina in which there is no continuous layer of nerve-fibres on the 

 inner surface, that is to say the ' yellow spot,' is the most sensitive to 

 luminous impressions. Under this notion the import of the ' rods ' 

 and their remarkable arrangement would l>e intelligible, and the 

 almost inexplicable correspondence in the size of the images of the 

 smallest distinguishable interspaces between two objects, with the 

 diameter of the ' rods ' and ' cones,' be placed in its true light. 



Vitreous Humour. (/'"</. 5, ; 7 rf.) The part next in order to 

 be described is the vitreous humour, behind which the retina is 

 disposed. It is a transparent fluid of semigelatinous consistence and 

 high refractive power, constituting about live-xixths of the bulk of 

 the globe. It consists of a fluid differing in no great degree from 

 water, contained in a celluluted structure called the hyaloid membrane 

 (fatal, glass), from its perfect translucency. The minute cells are 

 connected together ; for if the external part be punctured, the fluid 

 contained in them gradually drains away. This cellular structure is 

 so delicate and fragile that it is almost impossible to obtain it 

 separately; but the membranous partitions are rendered slightly 

 opaline by strong spirit or diluted acids, and may thus be made 

 evident. It is condensed into a membrane of a firmer consistence 

 upon the surface, which serves the general purpose of a containing 

 capsule for the vitreous humour, and is strong enough to cause it to 

 preserve its shape in some degree when the stronger tunics of the eye 

 are removed. There is a narrow tubular dimple of some depth in the 

 vitreous humour opposite the entrance of tip in rvr. lined by a 

 trumpet shaped production into it of the external membrane, called 

 the hyaloid canal. (Tig. 5.) It serves to transmit a branch of the 

 retinal artery and associated veins for the nourishment of the capsule 



of the lens in the footus, and perhaps also of the hyaloid membrane, 

 and of the substance of the lens itself. There can be no doubt that 

 the vitreous humour is secreted by the surfaces of the hyaloid cells, 

 but this fietal artery is the only vestige of a vascular arrangement 

 yet discovered in the part 



Fig. 7. 



This figure, from the work of '/Ann, represents the Vitreous Humour In Its 

 hyaloid membrane. The lens, imbedded in it* upper surface, la surrounded by 

 the canal of Petit. The dark border beyond in the plaited portion of the 

 hyaloid membrane called the zone of Zinn, stained with the pigment left by the 

 ciliary body, which in the natural state of the parts rents upon that portion of 

 the surface of the hyaloid. The outer edge of the rone i marked by a waved 

 outline, correnponding with the ora serrata of the ciliary body. 



a, crystalline lens ; *, canal of Petit ; c, zone of Zinn ; d, vitreous humour. 



The zone of Zinn, or zonula Zinnii, is that portion of the hyaloid 

 membrane which is continued to the border of the lens, where it 

 becomes blended with the capsule of that body. In doing this it 

 splits into two lamelhe, the posterior of which is blended with the 

 capsule of the lens, and the anterior connected with the ciliary pr<>- 

 cesses which ore attached to the capsule of the lens a little in front of 

 its margin. Between the two lamella; and the border of the lens there 

 is left a space surrounding the latter in an annular manner, and in a 

 transverse section of a triangular form. This is the canal of Petit 

 The darker colour of the zone of Zinn appears to depend on the 

 existence of pigment-cells belonging to the choroid, which are situated 

 principally in the folds in which the processus ciliares were contained, 

 and give the whole zone a striped aspect On the inner side of this 

 layer lies a single layer of clear very frequently pale polygonal 

 nucleated cells of 0-006'" 0-012'" in size, but which is never entire, 

 being always partially removed together with the ciliary processes on 

 which Henle and others have noticed it. 



Lens, or Crystalline Humour. (Figi. 5,/; 7, a; 8.) The crystalline 

 lens (KptioroAAor, crystal) is imbedded in a deep depression in the front 

 of the vitreous humour, a little nearer the nasal than the temporal side 

 of the globe. It has the form and function of a double convex lens. 

 The surfaces may be considered as portions of two unequal spheres, 

 the anterior being considerably flatter than the posterior. The 

 diameter of the sphere of which the former is a segment is about 

 eight-twelfths, of the latter five-twelfths of an inch. The thickness of 

 the lens, measured in the axis of vision, is about the sixth part of on 

 inch, and its transverse diameter is about twice that length. (I'i : /. I. 

 c c, d d.) In refractive power it is superior to the other ' 

 substances contained in the eye. Its consistence is gelatinous, 

 increasing in density from the circumference towards a central nucleus, 

 which has the tenacity of soft wax. It is composed of an infinite 

 succession of thin concentric lamiuic, arranged with the utmost regu- 

 larity one within another like the coats of an onion ; and every such 

 stratum or elliptic shell is made up of a series of exquisitely minute 

 fibres laid side by side, forming three septa like the cloves of an 

 orange, of which tin* Uimnling or cleavage planes diverge from tin' 

 axis of the lens at angles of 120 in the manner represented in tin- 

 annexed figure. (Fig. 8.) If the lens be hardened in strong spirit, 

 the result of this curious arrangement is, that it partly cleaves into 

 three portions mode up of layers which may be peeled off one after 

 another, each further separable to a certain extent into its component 

 parallel fibres. The spirit not only hardens the crystalline humour, 

 but renders it opaque ; and the same effect is produced by plnniriiiK 

 it in boiling water, as every one must have observed in the eyes of 

 dressed fish. In fact it consists chiefly of the transparent substance 

 called albumen found in eggs, and is coagulated by heat in the same 

 way. The lens is similarly constructed in the eyes of other Mammalia ; 

 and analogous, though not identical, arrangements are observed in 

 other classes. In fish the fibres we have spoken of are curiously 

 hooked together by fine teeth, resembling those of a saw. We chiefly 

 owe the discovery of these facts respecting the intimate structure of 

 the lens to the labours of Leeuwenhoek, Young, and Brewster, whose 

 writings may be consulted for much further interesting information 

 on the subject. The crystalline Ions is inclosed in a transparent and 

 highly clastic membranous capsule, represented in jig. 5 by a white 

 line, to the regulated pressure of wlii.l, the preservation of its true 

 shape, so important to the purposes of vision, is mainly attrili" 

 There it, no analogous structure in the body, except the internal 

 lining of the cornea already mentioned, which closely resembles it in 



