THE BALL OF THE EYE. 421 



Ruysch attributed two laminae to the choroid membrane in the 

 human subject, the internal of which was called after his name, 

 but the distinction is now generally abandoned. 



The Iris is a circular plane placed at the front of the^pho- 

 roides, and having, in its centre, a round opening called the 

 pupil (pupilla.) Its external circumference as stated is attached 

 to the ciliary ligament, and by it to the choroid coat, and is ex- 

 actly at the junction of the cornea with the sclerotica. Pro- 

 fessor Soemmering has bestowed much attention in ascertain- 

 ing whether this membrane is perfectly flat or somewhat con- 

 vex in front, and, by repeated observations, carefully made, 

 has assured himself that it is flat.* My own observations and 

 preparations have induced me to believe that in many cases it 

 will be found slightly convex in front, as Petit asserted more 

 than a century ago. In a moderate state of dilatation, its na- 

 sal or internal border is somewhat narrower than its external 

 or temporal. With the exception of its external circumference, 

 every part of the iris is void of any attachment; by which ar- 

 rangement it moves freely in the aqueous humour, so as to con- 

 tract or dilate the pupil, according 10 the quantity of light ad- 

 mitted upon the eye. 



The iris, with the exception of its central or pupillary cir- 

 cumference, where it is thinner than elsewhere, is much thick- 

 er than the choroid coat. The posterior face of the iris, some- 

 times called uvea, is covered in g ^i' abundance with pig- 

 mentum nigrum. When t us is removed by maceration, which 

 may be readily done, the membrane becomes semi-transparent. 

 Its anterior surface is the seat of the colour, which characterizes 

 every individual's eyes. There are but two of these colours, 

 light blue and orange, the predominance of one or the other of 

 which, assisted by the dark ground on the back of the iris, gives 

 the cast of hue to the ey^. i'he front surface, when examined 

 on the living eye wi h t ie microscope, is so ;n to be downy or 

 flocculent, and is traversed by filament forming an intertex- 

 ture, some of which are circular, . ers oblique; and others ra- 

 diated. This arrang nieat is remarkably u.si, cl in the eye of 

 the seal. 



* Icones Oculi Humani. 



VOL. II. 37 



