24 



considers to be merely for the purpose of connecting the sclerotic 

 and choroid anteriorly, as these tunics are also connected by cellu- 

 lar tissue posteriorly, but very loosely in their middle. 



The author next describes the manner in which these fibres may 

 be most easily displayed, and, as the sheep's eye answers best for 

 this purpose, his description is given in reference to it. The poste- 

 rior tw^o-thirds of the eye of the sheep being turned inside out, and 

 all the retina washed awT.y, a very thin portion of the tapetum is 

 raised and floated upon a glass slide, and a thin glass cover is then 

 placed upon it, without making the least pressure, which would, by 

 crushing the very soft primary fasciculi, render the transverse striae 

 irregular and confused. If in this state it be examined by the mi- 

 croscope, the muscular fibres, in consequence of their transparency, 

 and being covered by the membrana pigmenti, and some of the iri- 

 descent fibres of the tapetum, will be very indistinct and scarcely 

 distinguishable; but if a small portion of acetic acid, or some aque- 

 ous solution of chlorine, be brought in contact with it, the membrana 

 pigmenti, and especially the iridescent fibres, will instantly shrink, 

 and, becoming transparent, disappear, and the fasciculi of the cho- 

 roid muscle will come into view. Should the acetic acid have been 

 too strong, the transverse marking will be very faint and scarcely 

 discernible ; but if the acid be washed away with water, or, what is 

 better, a w^eak solution of any saline substance, such as chloride of 

 potassium or sodium, iodide of potassium, Sec, the transverse mark- 

 ing will become very distinct ; should the solution be strong, the 

 colour of the iridescent fibres will be reproduced. In the eye of 

 those animals whose tapetum is scaly, as in the Cat, or where the 

 whole of the choroid is lined by black pigment, as in the human 

 subject, the acetic acid is of no use, and the muscle is displayed 

 with much greater difficulty; the author, however, states that he 

 has succeeded in displaying this muscle in the choroid of the Horse, 

 the Cat, and in that of the human eye"''. 



The author next considers the action of the choroid muscle. This, 

 from the manner in which the muscle embraces the vitreous humour, 

 is, in his opinion, to compress this humour and carry the lens for- 

 wards. But this, he observes, considering how the capsule of the 

 lens and the ciliary processes fi.ll up the posterior chamber of the 

 aqueous humour, cannot be efi'ected without displacing some of the 

 fluid in that chamber ; and he concludes that the aqueous humour 

 in the posterior chamber being pressed by the capsule of the lens 

 (forced forwards by the action of the choroid muscle) against the 

 ciliary processes, forces the blood out of their vessels into the cho- 

 roid veins, and thus enlarges this chamber at its circumference, 

 •whilst it diminishes it from behind forwards, or, in other words, 

 moves the lens forwards. The author then observes, that, as the 

 effect of the action of the choroid muscle is to separate the choroid 

 coat from the sclerotic, all pressure is taken off the choroid veins at 

 the time it is made upon the ciliary processes, and therefore that 



* The author has found, since the paper was read, that a solution of citric acid, 

 one drachm of acid to the ounce of water, is the best substance for rendering the 

 iridescent fibre transparent. 



