Chap. 41.] The Iris and Pupil. 387 



rays of light are admitted to the internal parts of the 

 eye. 



The tunica choroides is defcribed by fome authors 

 as confifting of two laminas. This defcription, how- 

 ever, applies much better to the eyes of fome animals, 

 particularly to thofe of fheep, than to thofe of man. 

 Thofe who fuppofe the choroides to confift of two 

 laminae, defcribe the external one as terminating at 

 the ciliary ligament, and the internal one as extending 

 further to form the iris. This iris itfelf is defcribed 

 as confifting of two laminas, and it is very certain that 

 two fets of fibres may be obferved, Thefe are fup- 

 pofed to be mufcular, and from the mobility of the 

 iris there feems no reafon to doubt of their being really 

 fo. Some of the fibres are orbicular, and lie round 

 the pupil j others are ftrait, and extend from the cir- 

 cumference of the iris to its center. The iris has mo- 

 tions of fuch a nature, that the pupil is contracted on 

 the approach of a ftrong light, and is dilated in pro- 

 portion as the light is lefs vivid. By this admirable 

 yet fimple contrivance, the eye adapts itfelf to the dif- 

 ferent proportions of light to which it is expofed. If 

 the pupil was always as much contracted as it is when 

 expofed to the light of noon day, a weaker light, 

 fuch as that of the moon, could not be admitted with 

 fufficient freedom to anfwer any ufeful purpofe. On 

 the contrary, if the pupil was immoveably dilated, w 

 might take advantage of the fcattered rays of light, 

 but Ihould be diftrefTed and blinded by the glorious 

 effulgence of the mid-day fun. When a flrong light v 

 fucceeds to darknefs, we are under a neceffity of doling 

 the eye-lids, or of turning away the head, till the pupil 

 has been accommodated to the change by the con- 

 tractile powers of the iris. 



The choroid coat is internally covered with a flimy 

 C c 2 fubftanc? 



