THE EYE AS AN OPTICAL INSTRUMENT. 511 



the pupil : so its second coat does not, like the outer one, 

 completely envelop the eyeball. In the iris is a ring of plain 

 muscular tissue encircling the aperture of the pupil : when its 

 fibres contract they narrow the pupil. Radial fibres can be 

 found passing from the ring to the outer edge of the iris, 

 and they have been supposed to be muscular and concerned 

 in dilating the pupil. They are probably merely elastic and, 

 being stretched when the circular muscle contracts, by 

 mere physical elasticity dilate the pupil when the muscle 

 relaxes. The circular or sphincter muscle appears to be 

 normally in a state of tonic contraction; this is increased 

 by impulses travelling in fibres of the third cranial nerve 

 and is diminished or inhibited by impulses travelling along 

 fibres of the sympathetic, which, however, have their origin in 

 the medulla oblongata and run down the spinal cord to the 

 lower part of the neck, where they pass out in anterior spinal 

 nerve-roots to reach the sympathetic. The pigment in the 

 iris is yellow, or of lighter -or darker brown, according to the 

 color of the eye, and more or less abundant according as the 

 eye is black, brown, or gray. In blue eyes the pigment is 

 confined to the deeper layers, and modified in tint by light 

 absorption in the anterior colorless strata through which the 

 light passes. 



The third coat of the eye, the retina, 15, is its essential 

 portion, being the part in which the light produces those 

 changes that give rise to impulses in the optic nerve. It is 

 a still less complete envelope than the second tunic, extend- 

 ing forwards only as far as the commencement of the ciliary 

 processes, at least in its typical form. It is extremely soft 

 and delicate; and, when fresh, transparent. Usually when 

 an eye is opened the retina is colorless; but when the eye has 

 been cut open in faint yellow light and the exposed retina 

 quickly examined in white light it is seen to be purple. The 

 coloring substance (visual purple) very rapidly bleaches when 

 a dead eye is exposed to daylight. On front or inner surface 

 of the human retina two special areas can be distinguished in 

 a fresh eye. One is the point of entry of the optic nerve, 16, the 

 fibres of which, penetrating the sclerotic and choroid, spread 

 out in the retina. At this place the retina is whiter than 

 elsewhere and presents an elevation, the optic mound. The 

 other peculiar region is the yellow spot (macula luted], 18, 

 which lies nearly at the posterior end of the axis of the eye- 



