



THE BALL OF THE EYE. 425 



of the choroid coat can be seen in animals destitute of pigmen- 

 tum nigrum. It is composed of two laminae, of which the ex- 

 ternal is nervous, and the internal, or that next to the vitreous 

 humour, is formed of a fine reticulated cellular membrane with 

 blood vessels running through it. The external lamina may 

 be removed by a camel's hair-pencil, or by slight putrefaction 

 and washing, so as to leave the internal entire. The cele- 

 brated John Hunter succeeded, however, in separating the two 

 laminse fairly from each other, and preserving them, so as 

 to show their difference. This specimen may be considered 

 unique, and every way deserving of the source from which it 

 proceeded.* 



Exactly in the axis of the eye, or at its centre, posteriorly, 

 consequently, about a line and a half from the outer side of the 

 bulb of the optic nerve, Soemmering discovered, in 1791, a 

 yellow spot (Macula flava) of a line in diameter, with a small 

 hole in its middle, made by a deficiency of medullary matter. 

 From the optic nerve there goes, towards the foramen, a small 

 fold of the retina, pointed at its internal end, and obtuse or bi- 

 furcated externally. Unless the eye be fresh, these things 

 cannot be seen distinctly, for the evaporation of the aqueous 

 humour causes a collapse or wrinkling of the retina, which 

 obscures them. But, in a perfectly fresh eye, which is well 

 managed, they may be seen both from before and behind. It 



* The fact was communicated to me by the late Dr. Physick, who studied un- 

 der Mr. Hunter, and frequently saw the preparation. 



The Retina is said by Langenback to have three coats. 1st. An external gra- 

 nular coat. 2. Ehrenberg's filamentous nervous coat: and 3d. A vascular coat, 

 consisting of blood vessels, united into a delicate membrane by much cellular 

 substance. Whether the filaments of the nervous coat be plain, cylindrical, or 

 nodulated ones, is unsettled. The granular layer terminates at the macula flava 

 by a sharp edge; and it ends also at the border of the ciliary ligament. Trevi- 

 ranus, also, admits the division of the optic nerve into cylinders, and of extreme 

 minuteness. The granular layer is said by Gottshe not to be found in recent 

 eyes ; he also says that, a perfectly recent retma has the appearance of a thatched 

 roof; that is little cylinders, the ends of which project out, like a staff. Michaelis 

 admits the division of the retina into three layers, as above, and adds a fourth, it 

 being the tunica Jacobi. The internal layer he considers as serous. The fasci- 

 culi of the nervous coat are best shown by means of the spirit of creosote. The 

 filaments he estimates at a diameter of the 5 Vo of a linc ' The seroqs c at ^ 

 best seen by the use of diluted sulphurig ac>d s Muller's Report on Nervous Sys- 

 tem, p. 237. 



37* 



