402 ELEMENTARY PHYSIOLO(;Y less. 



the front part of the sclei'otic, its inner surface becomes 

 raised up into a number of longitudinal ridges, with 

 intervening depressions, like the crimped frills of a lady's 

 dress, terminating within and in front by rounded ends, 

 but passing, externally, into the iris. These ridges, 

 which when viewed from behind seem to radiate on all 

 sides from the lens (Figs. 128, c, and 126, 5), are the 

 above-mentioned ciliary processes. 



The iris itself (Figs. 126, 7, and 128, o, 6) is, as has 

 been already said (p. 303), a curtain with a round hole in 

 the middle, the pupil, provided with circular and radiating 

 unstriped muscular fibres, and capable of having its 

 central aperture diminished or enlarged by the action of 

 these fibres, the contraction of which, unlike that of 

 other unstriped nmscular fibres, is extremely rapid. The 

 edges of the iris are firmly connected with the capsule of 

 the eye, at the junction of the cornea and sclerotic, by 

 the connective tissue which enters into the composition 

 of what used to be called the ciliary ligament. The 

 hinder surface of the iris is covered with cells containing 

 a black pigment, similar to that of the choroid coat, and 

 the different colours of eyes depend partly on the varying 

 amount and distribution of pigment in these cells, but 

 chiefly on pigment cells imbedded in and scattered 

 throughout the substance of the iris. Unstriped muscular 

 fibres, having the same attachment in front, spread back- 

 wards on to the outer surface of the choroid, constituting 

 the ciliary muscle (Fig. 126, 4). If these fibres 

 contract, it is obvious that they will pull the choroid 

 forwards ; and as the frame, or suspensory ligament of 

 the lens, is connected with the ciliary processes (which 

 simply form the anterior termination of the choroid), 

 this pulling forward of the choroid brings about a 

 relaxation of the tension of that suspensory ligament, 

 which, as we have just said, is in a resting condition 

 stretched somewhat tight, keeping the front of the lens 

 flattened. 



