422 NERVOUS SYSTEM. 



The power which the iris has of dilating the pupil when 

 there is but little light, and of contracting it when there is 

 much, has induced many anatomists to think that it is formed 

 of muscular radiated fibres, which by their contraction produce 

 the first motion, and of circular ones which produce the last. 

 Among these anatomists may be mentioned, Ruysch, Morgag- 

 ni, Zinn, Sabatier. Ruysch asserted that the radiated fibres 

 extended from the greater circumference of the iris to the pupil, 

 and were fixed there by very.delicate tendons. The late Doc- 

 tor Monro, of Edinburgh, has described particularly the circu- 

 lar fibres, and a preparation of the bullock's eye which belonged 

 to him is still exhibited there, where these fibres are found around 

 the margin of the pupil. The several fibres can only be seen 

 distinctly, when the pigmentum nigrum is washed away. De- 

 mours and Meckel deny the existence of the radiated fibres. 

 The late distinguished Professor Wistar taught that the con- 

 traction of the pupil was produced by circular fibres, and the 

 dilatation of it by its elasticity. In objection to this, the late 

 Dr. Physick remarks, that as elasticity is as much a property 

 of dead as of living matter; in death, therefore, we should al- 

 ways find the pupil dilated from the want of active contraction 

 in the circular fibres; also, in cases of concussion of the brain, 

 where there is a sudden loss of sensibility and of muscular mo- 

 lion, the pupil should be invariably dilated; but the fact is, that 

 the pupil remains just in the same state that it was at the mo- 

 ment of the accident.* 



Notwithstanding the extreme sensibility and mobility of the 

 iris on the admission of light, one is occasionally astonished to 

 find it not contracting when instruments are applied to it, as I 

 have had an opportunity of twice observing, upon the removal 

 of a considerable portion of it, in making an artificial pupil for 

 opacity of the cornea. In these cases, upon the letting out of 

 the aqueous humour, it became quite as flaccid as we are ac- 

 customed to see it in our dissections. The same remark has 

 been made by Mr. now Sir Charles Bell. 



The Blood Vessels of the Iris are principally branches of the 

 Long Ciliary, which have been alluded to. Each of the two 



* It would appear that the question of the muscularity of the iris has been 

 settled almost conclusively by Mr. Bauer. See Ph. Trans, for 1822. 



