

SENSE OF SIGHT. 197 



of objects to the brain, the orbicular muscle relaxes, and the 

 radiated one contracts, so as to enlarge the pupil to its full 

 extent. Between these two muscles is a middle layer, com- 

 posed of a net- work of blood-vessels and nerves. 



Fig. 4. 



The cut marked I. represents the iris magnified as seen from the 

 front, showing the radiated muscle. 



Cut II. is a back view of the same showing the orbicular muscle. 



13. The iris then, regulates the quantity of light passing 

 through the pupil. The pupil during sleep is in an intermediate 

 state ; in inflammation of the brain it is generally contracted ; 

 but when the functions of the brain are interrupted by nar- 

 cotic poisons, the pressure of effused fluid, a tumour, or any 

 other cause, the pupil is dilated. The bella donna, or deadly 

 night-shade, is employed by surgeons for this purpose, when 

 they wish to couch, or perform other operations on the eye. 

 The contraction of the pupil is readily seen by holding a 

 candle close to the eye of a person, and then withdrawing it 

 slowly ; or by directing a person to look at a very near ob- 

 ject, and then at one more distant. Some persons can dilate 

 and contract the iris at pleasure. I possess this faculty to 

 a very great extent, and was conscious of it before I ac- 

 tually tried the experiment. The motion of the iris is not at 

 all associated with that of the lids, as Walker suggests in his 

 late excellent work on the Philosophy of Sight, the lids 

 remaining the whole time perfectly stationary. I am, when 

 contracting the iris, sensible of an effort similar to that of 

 examining a very near object, and when dilating it, like that 

 17* 



