DISSECTION OF THE EYE. 447 



Inches. 



Length of the optical axis, . . . 0.91 



Vertical cord of the cornea, . . . 0.45 



Versed sine of the cornea, . . . 0.11 



Horizontal cord of the cornea, . . . 0.47 



Diameter of pupil as seen through ditto, 0.27 to 0.13 



Distance of iris from cornea, . . . 0.11 



Distance of iris from anterior surface of lens, 0.02 



Radius of anterior surface of lens, . • 0.30 



Radius of posterior surface of lens, . . 0.22 



Principal focal distance of lens, . . . 1.7.3 

 Distance of the centre of the optic nerve from the 

 central fold, (jyellow spot, limhum luteum,) in 

 the axis of vision, . . . .0.11 



Range of eye, or diameter of the field of vision, 1.100 



It will be very beneficial to every student of anatomy to dissect this delicate 

 organ himself, as he will thereby acquire more accurate and precise ideas of its 

 structure than he can possibly obtain from the ordinary demonstrations. 



The eyes of sheep and oxen will serve very well for beginners, as many of 

 them will be required, and they can be easily obtained. They resemble the 

 human eye in many respects ; and the circumstances in which they differ may 

 be readily ascertained by the diligent student ; especially if he dissects the 

 human eye afterwards. 



For this purpose, it will be requisite to have forceps, finely pointed ; with 

 knives and scissors that are equally delicate in structure ; and also a pair of 

 strong scissors, bent like those in the cases of pocket instruments. 



"When removing the exterior parts, it will be useful to preserve a portion of 

 the optic nerve, to take hold of. 



The dissection of the coats of the eye can be best performed when the eye is 

 placed in a vessel of water ; which should be shallow or deep, according to the 

 diflferent stages of dissection. 



In some dissections it will be serviceable to form a bed of jelly, to support 

 the eye, as proposed by Mr. C. Bell. 



As the first process in tliis operation, the sclerotic coat can be readily separa- 

 ted from the choroides, and the cornea with it. 



After examining the external surface of the choroides, the ciliary ligament, 

 and the iris, the iris may be peeled off from the choroides. 



The choroides, by a careful process, may then be separated from the retina, 

 which it leaves surrounding the vitreous humor. The ciliary processes, being 

 a part of the choroides, come away with it. 



The preparation being in water may now be suspended by the optic nerve. 

 It consists of the vitreous and crystalline humors and the retina. The retina, 

 originaiing from the optic nerve, adheres anteriorly, also, with so much firm- 

 ness, that it will support the part enclosed. 



