.190 



EYE. 



the retina appears to be forced or drawn at this 

 point into the vitreous humour to the depth 

 of about a twelfth of an inch, the entire fold 

 being something more than an eighth in length. 

 At first there is little or no appearance of a 

 hole, but after the eye has remained for some 

 time in the water, the fold begins to give way, 

 and a small slit makes its appearance, which 

 gradually widens, and assumes the appearance 

 of a round hole. This hole is large in pro- 

 portion to the degree to which the fold has 

 yielded ; and when the fold totally disappears, 

 as it sometimes does, the transparent point 

 gives the appearance which Sommerring re- 

 presents, of a hole with a yellow margin. If, 

 instead of making the examination in this way 

 from the outside, we view this part through 

 the vitreous humour, the appearance of the 

 hole is more remarkable ; but still that part of 

 the retina is evidently projected forward be- 

 yond the level of the rest of that membrane. 

 In the eye of a young man, which I had an 

 opportunity of examining under peculiarly 

 favourable circumstances, within five hours 

 after death, I noticed the following appear- 

 ances. The cornea and iris having been cut 

 away, and the lens removed from its situation, 

 I placed the part in water, beneath one of the 

 globular glasses, and held it so as to allow the 

 strong light of a mid-day sun to fall directly 

 upon it ; when the retina to the outside of the 

 optic nerve presented unequivocally the ap- 

 pearance of being drawn or folded into the 

 form of a cross or star, with a dark speck in 

 the centre, surrounded by a pale yellow areola. 

 I further satisfied myself of the prominence 

 of the fold by holding a needle opposite to it, 

 while the light shone full upon it, a shadow 

 being thus cast upon the retina which deviated 

 from the straight line when passed over the 

 situation of the fold. To ascertain whether 

 there is actually a hole in the retina, or merely 

 a deficiency of nervous matter at this point, 

 I allowed the eye to remain for some days in 

 water, until the connexions of the parts began 

 to give way. I then introduced a small probe 

 between the retina and vitreous humour, the 

 part still remaining in water, and bringing the 

 blunt point of the instrument opposite the 

 transparent spot, attempted to pass it through, 

 but found I could not do so without force 

 sufficient to tear the membrane. I also re- 

 moved the nervous matter by maceration and 

 agitation in water, and on floating the vascular 

 layer, found that I could no longer ascertain 

 where the spot had originally existed, there 

 being no hole in the situation previously occu- 

 pied by the transparent speck." 



It is remarkable that the foramen of Sommer- 

 ring has not been found in the eyes of any of 

 the mammalia except those of the quadrumana, 

 in some of whom it has been detected by Home, 

 Cuvier, and others, but the extent to which it 

 may be traced in this tribe has not been satis- 

 factorily ascertained. Dr. Knox, in a paper in 

 the Memoirs of the Wernerian Natural History 

 Society, announces the discovery of its existence 

 in certain lizards. In the lacerta superc'diosa he 

 says, " the retina is very thick, and somewhat 



firm and opaque. Where the optic nerve enters 

 the interior of the eye-ball, there is a distinct 

 marsupiuw or black circular body, proceeding 

 forwards apparently through the centre of the 

 vitreous humour. Anteriorly, somewhat supe- 

 riorly and towards the mesial line or plane, we 

 perceive, on looking over the surface of the 

 retina which regards the vitreous humour, a 

 comparatively large transparent, nearly circular 

 spot, through which may be distinguished the 

 dark-coloured choroid. Close to this is gene- 

 rally placed a fold or reduplication of the retina, 

 which is in general remarkably distinct. This 

 fold or folds, (for there are more than one) 

 either proceed from the transparent point 

 towards the insertion of the optic nerve, or 

 close to it. Sometimes the fold seems, as it 

 were, to lie over the transparent point, and 

 partly to conceal it from view ; or the point is 

 formed in the edge of the fold itself, as in apes, 

 but in general the fold runs directly from 

 the insertion of the optic nerve upwards and 

 inwards, pressing very close to the edge of the 

 foramen centrale." The foramen was also 

 seen in the lacerta striata, lacerta calotes, and 

 others, while it was not to be detected in the 

 gecko, crocodile, and some others. It was also 

 subsequently discovered in the chameleon. 

 The annexed figures represent the foramen of 

 Sommerring in the human eye. A, shews the 

 retina expanded over the vitreous humour : on 

 the right is the place from which the optic 

 nerve was cut away, and from which the ves- 

 sels branch out : on the left is the foramen of 

 Sommerring, represented by a black dot sur- 

 rounded by a dark shade. B, shews the retina 

 with a portion of the optic nerve. The exter- 

 nal membrane is turned down as in the pre- 

 ceding representation of the same structure in 

 the sheep's eye, and the foramen of Sommer- 

 ring, instead of a distinct hole, presents the 

 appearance of a fold or depression with elevated 

 sides. The wood-engraving does not admit of 

 the delicacy of finish necessary to express per- 

 fectly this condition of the part. 



Fig. 114. 



A. 



There is no part of the anatomy of the eye 

 respecting which there has been so much diver- 

 sity of opinion as the anterior termination of 

 the retina. It has already been stated that it 

 extends to the posterior extremities of the 

 ciliary processes, where it is discontinued, pre- 

 senting an undulating edge corresponding to 

 the indented margin of this part of the corpus 

 ciliare. Some assert that it extends to the mar- 

 gin of the lens, others that it is the vascular 



