



I-.YK. 



the retina nearer ami nearer to the entrance of the nerve an he goes 

 backward. At length when he hu reached the distance of about 

 64 feet from the wall, the right-hand object will suddenly disappear, 

 and remain invisible (the observer (till retreating) till be ban gained a 

 distance of about eight feet. During this period the spectrum has 

 been pasting over the circular aperture in the choroid through which 

 the nerve enters. The insensible portion of the retina in found to 

 extend horizontally over fir* degrees and a half of the angular range of 

 vision. The eyes are generally unequal in power, and the experiment 

 succeeds best in the weaker organ, in which the obscuration is more 

 sudden and complete. In the experiment previouly mentioned, 

 showing the distribution of the central artery of the retina, the 

 surface of the choroid ia faintly illuminated through the transparent 

 nervous expanxion by what is called the dispersion of port of the 

 light admitted through the pupil ; but the rays thus scattered are 

 locally intercepted by the opaque blood contained in the minute 

 branches of the artery ; hence, after several repetitions, when the eye 

 hill become accustomed to neglect the taper, and attend to the fainter 

 internal illumination, the shadow of the vascular net-work upon the 

 choroid becomes perceptible in dark lines. 



Apparent Direction of Objects seen obliquely. A body in motion, 

 as a ball, striking the surface of another, impresses it in a lino perpen- 

 dicular to the surface at the point of impact. This rule appears to 

 hold good with respect to the action of light upon the retina. Indeed 

 if impressions of any kind be made upon it, the sensation is that of 

 light, and the direction suggested is that of a line joining the centre 

 of the sphere of which the retina forms a part with the point im- 

 pressed, in other words, a line perpendicular to it. This may be 

 shown in several ways : if we excite the nerve by pressing far back 

 upon the eyeball with the finger-nail, especially if the eye be closed 

 or light otherwise excluded, a bright ring appears to be seen in a 

 diametrically opposite quarter. 



Erect Vision. If the sclerotic and choroid be carefully removed 

 under water from the back of an eye, an inverted picture of any 

 object held before the cornea is seen upon the now milky surface of 

 the retina. Hence the celebrated question raised in the age of philo- 

 sophical barbarism, how is it that we see objects erect when the 

 image on the retina is inverted ? The question is an idle one, which is 

 perhaps hardly worth answering. The mind judges of the apparent 

 place of objects or of parts of an object by the direction of the 

 impressions made upon the retina, not by the part of it which may 

 happen to be affected by these impressions. The shadow of the 

 central artery U an example of an impression necessarily received 

 always upon the same parts; yet the apparent, or in other words 

 the relative, place of the shadow will be found to vary with every 

 movement of the eye. 



Single Vision. Another question, not so trivial as the last, has 

 been raised with respect to single vision with two eyes, as the im- 

 pression must be twofold. But perhaps it will not require an answer 

 if the reader will try to imagine double vision of the same object, 

 or rather of the same point, for the question resolves itself into that. 

 Let the two supposed images approach each other, still remnininp 

 double, till they are in contact Another step in the imaginary 

 approximation, and they are one. The truth is, that both eyes see 

 the object in the same place ; and as two images, no more than two 

 material substances, can occupy the same place at the same time, the 

 impressions coincide and are single. 



On the Development of the Organ of Vision. The following remarks 

 on this subject are from Professor Kblliker's 'Manual of Human 

 Histology:' 



" The eyeball is not developed from a single point as a whole, but 

 arises from the conjunction of formations, proceeding on one side from 

 the central nervous system, on another from the skin, and thirdly 

 from the parts lying between the two. 



" In the Chick the primitive ocular vesicles arise before the com- 

 mencement of the second day, from the primitive cerebral vesicle or 

 the anterior cerebrum, in the form of two protrusions at first sessile, 

 but afterwards baring a hollow peduncle the rudiment of the optic 

 nerve. At the beginning of the third day the formation of the lens 

 commences, from the skin of the (ace covering these vesicles by the 

 thickening on the inntr aspect and inversion of the epidermis, in con- 

 sequence of which the anterior wall of the primitive ocular vesicle is 

 also inverted, and become* applied to the posterior wall, o that the 

 cavity of the vesicle is wholly obliterated. 



" Now, at first this secondary ocular vesicle encompasses the lens, 

 which in the meantime has been separated by constriction from the 

 epidermis, and comes into exact apposition with it beneath ; like a 

 cup subsequently however the vitreous body is developed between 

 the two in a special new cavity. How the Utter is formed has not 

 yet been ascertained, although, as Scholer observes, it is most probable 

 that it also grows in from the skin, in fact from the region below and 

 behind the lens, and participates with the latter in the inversion of 

 the primitive ocular vesicle. According to Remak, the retina is 

 formed from the inner thicker wall of the inverted or secondary ocular 

 vesicle, and from toe outer and thinner, the choroid, from the anterior 

 border of which the iris is not produced till afterwards. The scle- 

 rotic and cornea are applied from without upon the eyeball thus con- 

 stituted, the former being to some extent a production of th skin. 



An interesting phenomenon is presented iti the vessels existing in the 

 foetal eye, even in the transparent media. The vitreous body on its 

 outer surface, between the hyaloid membrane and the retina, presenti 

 a tolerably wide meshed vasculnr plexus, which is supplied by 

 branches of the arteria centralis retin:r, _! VCM off from it at its entrance 

 into the eye, and anteriorly, at the border of the lens on the zmil:i 

 Xinnii forms a vascular circle, the circulus arteriosus Mancngnii, from 

 which again vessels are given off to the membrana capsulo-pupillarU 

 presently to be described. Besides this, a special arteria hyaloidea, 

 also derived from the central artery of the retina, runs in the so 

 cunalis hyaloidnis, in a straight line through the vitreous body 

 lens, and ramifies in the most elegant arborescent manner at v.ry 

 acute angles in a membrane closely applied to the posterior wall of 

 the lenticular capsule. This is nothing else than a portion of an 

 externally vascular capsule which at first very closely surrounds the 

 lens, and in its anterior walls is supplied by the continuation of the 

 hyaloid artery coming round the border of the lens towards the front, 

 with which branches of the circulus arteriosus Mascagnii and of the 

 anterior border of the uvea are connected. Afterwards, wlieu the 

 lens retreats from the cornea, with which it is at first in close appo- 

 sition, and the iris bulls out from the border of the uvea, the anterior 

 wall of the vascular lenticular capsule is divided into two portions, 

 one central and anterior, which, arising from the border of the iris, 

 and connected with that membrane by vessels, closes the pupil, the 

 membrana pupillaris; and another external and posterior, extending 

 backwards from the same points on the border of the lens, the mem- 

 brana capsulo-pupillaris. The latter becomes more and more distinct 

 as the iris and aqueous chambers are developed, and the lens retreats 

 until at last it represents a delicate membrane stretching across the 

 posterior chamber. 



" The venous blood from all these parts is returned through the 

 veins of the iris and from the outer surface of the vitreous body, also 

 through those of the retina, and perhaps through a vena-hyaloidva 

 said to take the same course as the lytery, but of the existence of 

 which many authors doubt, and which I have never myself 

 With respect to the genetic import of the vascular capsule, n> 

 has as yet been ascertained. I find it to be composed of a homoge- 

 neous tissue, with a few scattered cells, and regard it as a structure 

 corresponding to the cutis which in the formation of the lens is 

 detached from the skin, together with a portion of the epidermis, and 

 remains in the eye. The vitreous body then may lie understood as 

 modified subcutaneous connective tissue a supposition not at all 

 incongruous with the observations above adduced, and the more so 

 because, as I have shown, all the subcutaneous connective tissue of 

 the foetus is at one time perfectly gelatinous, and, like the cnanu -1 

 organ, which also belongs to the same tissue, in specie strikingly 

 resembles the vitreous body in aspect and consistence. 



"Concerning the histological development of the eyes, the following 

 only need be remarked. At an early period they consist in all tlu-ir 

 parts of formative cells of uniform size, which in process of time are 

 metamorphosed into the various tissues. In the fibrous coat in the 

 second and third month the cells are developed in the mode already 

 described into connective tissue, and at the same time the distinction 

 is set up between the cornea and sclerotic, which are at first exter- 

 nally exactly alike, and constitute only a single membrane. In the 

 uvea the cells are for the most part employed in the formation of 

 vessels ; another portion goes to the formation of the inner and outer 

 pigment layers, pigment granules being deposited in th.-m at the com- 

 mencement of the third month, whilst another U transformed into 

 muscles, nerves, the cpithelia and connective tissue of these mem- 

 branes. The development of the nerve-cells and of the so-termed 

 'granules' from embryonic cells, may be readily traced. I have 

 observed the same thing also with respect to the 'cones,' and I think 

 that in the Frog it may be assumed with respect to the 'rods' like- 

 wise, that they are nothing but elongated cells ; whil.st in the M 

 malia the formation of the rods and of the nerve-fibres themselves, 

 has not yet been traced. The lens, lastly, is originally composed 

 entirely of cells, which in course of time arc transformed into the tubes. 



" The precise nature of the processes attending these changes has 

 not yet been investigated, although I agree with H. Meyer in 1 1> 

 elusion, that, since the tubes, both in the fcctus and child, present only 

 a single nucleus, each of them is developed out of a single cell. These 

 nucli-i, taken as a whole, constitute a thin lav < from the 



borders of the lens, through the middle of iti anterior half, nnd 

 slightly convex in front ('nuclear /MH. . Meyer); the nuclei being 

 smaller in the interior portions, and, as it were, in progress of solu- 

 tion, whence it may certainly be concluded tli.it the leu* increases by 

 the apposition of thin layers from without. The formative-cells of 

 the tubes of the lens are those which exist on the anterior half of the 

 capsule, and the starting-point of the formation of the lenticular 

 elements, according to my observation, is the entire anterior surface 

 and the border of the organ. Nucl'-i in the tubes even in tho 



lens of the adult, as was known to Harting, though only at its margin." 



(Knlliker, Manna/ ../ Human Hiriolwjy, translated by Busk and 

 Huxley for Sydcnham Society; Valentin nf BMMM /'/<//- 



tiology ; Todd and Bowman, Phyninlvijical Anatomy; Carpenter, 

 Princij'ir* nf l'hi/ti:,it,'in : .lones, Aclonian Pria Euay on the Kyc.) 



KYEBUIOHT. [KU 



