188 



EYE. 



without any connection, and the medullary 

 fibres in contact with a coloured mucus re- 

 tained in its situation by its consistence alone. 

 This account is totally at variance with the 

 general laws of the animal economy ; in no 

 instance have we parts, so dissimilar in nature, 

 in actual contact: wherever contact without 

 connection exists, each surface is covered by a 

 membrane, from which a fluid is secreted; 

 and wherever parts are united, it is by the 

 medium of cellular membrane, of which se- 

 rous membrane may be considered as a mo- 

 dification. If the retina be merely in contact 

 with the vitreous humour and choroid, we 

 argue from analogy, that a cavity lined by 

 serous membrane exists both on its internal 

 and external surface : but this is not the feet. 

 In the eye a distinction of parts was necessary, 

 but to accomplish this a serous membrane was 

 not required ; it is only demanded where great 

 precision in the motion of parts was indis- 

 pensable, as in the head, thorax, and abdo- 

 men; a single membrane, with the interpo- 

 sition of cellular substance, answers the pur- 

 pose here. By this explanation we surmount 

 another difficulty, the unphilosophical idea 

 of the colouring matter being laid on the 

 choroid, and retained in its situation by its 

 viscidity, is discarded ; as it follows, if this 

 account be correct, that it is secreted into the 

 interstices of fine cellular membrane here, 

 as it is upon the ciliary processes, back of the 

 iris, and pecten, under the conjunctiva, round 

 the cornea, and in the edge of the membrana 

 nictitans and sheath of the optic nerve in many 

 animals. Dissections are recorded where 

 fluids have been found collected between the 

 choroid and retina, by which the structure of 

 the latter membrane was destroyed ; the ex- 

 planation here given is as sufficient to account 

 for the existence of this fluid, as that which 

 attributes it to the increased secretion of a 

 serous membrane." 



The membrane is represented as it exists in 

 the eye of the sheep, in the annexed figure, 

 from my paper in the Medico-Chirurgical 

 Transactions. 



ig. 111. 



Mr. Dalrymple, in his valuable work on the 

 anatomy of the eye, takes a different view of 

 the arrangement of this part of the retina : 



he says : " From observations made on the 

 human eye, in connection with other expe- 

 riments on the eyes of animal, I am induced 

 to consider it as a double reflected serous mem- 

 brane. I was first led to take up this opinion 

 in the year 1827, by the accidental observation 

 of a very delicate membrane, which lined and 

 was adherent to the entire choroid. Having 

 minutely injected the eye of a sheep, I made 

 a vertical transverse section through the sclero- 

 tic, choroid, and retina, which last membrane, 

 with Jacob's tunic, properly so called, and the 

 vitreous body I removed. I then placed the 

 remaining portion of the eye in dilute spirits 

 of wine, intending to preserve it for the ex- 

 hibition of the tapetum, which in this instance 

 was remarkably beautiful. A few minutes 

 after its immersion the tapetum lost to a con- 

 siderable extent its brilliant hue, and I re- 

 moved it from the glass to wash from its sur- 

 face some deposit, which I thought might 

 have obscured its polish. In doing this, how- 

 ever, I detached a delicate membrane, mi- 

 nutely filled with injection, and this membrane 

 it was which on being placed in the spirit, 

 became slightly opaque and produced the effect 

 alluded to ; for the tupetum thus denuded in- 

 stantly recovered, and still retains its bril- 

 liancy." 



The inference that the membrane in ques- 

 tion is a double reflected serous membrane is 

 certainly more in conformity with analogy than 

 the assumption that it is a single layer, but this 

 uniformity in nature's operations has been too 

 much insisted upon. I have above stated my 

 reasons for considering it a single layer, and 

 not a double serous membrane ; and I should be 

 inclined to think that the layer which Mr. Dal- 

 rymple found adhering to the choroid was the 

 membrane itself, which had not come away 

 with the retina and vitreous humour, as I have 

 found sometimes to happen, did not Mr. Dal- 

 rymple further state that he has " in his pos- 

 session a preparation, which does most dis- 

 tinctly shew the double portions of this mem- 

 brane ; one lining the choroid, the other 

 reflected over the pulpy structure of the retina." 

 Mr. Jones, in the work formerly alluded to, 

 gives the annexed representation of the mem- 

 brane as it appears when fie. 112. 

 highly magnified. Fig. 11;! 



is a representation of the 

 membrane by Mr. Bauer, 

 magnified fifty diameters, from 

 the Philosophical Transactions 

 for 1822. 



In the centre of the retina, and consequently 

 in the axis of vision, about an eighth of an 

 inch from the entrance of the optic nerve, a 

 very remarkable condition of structure exists. 

 This is a small point destitute of cerebral or 

 medullary fibres, appearing like a hole in the 

 membrane, and hence called the foramen of 

 Siimmerring, from the distinguished anatomist 

 who discovered it. This point is surrounded 

 by a yellow margin, and the retina is here also 

 .puckered into a peculiar form of fold. Som- 

 merring, in the Commentationes Societatis 



