550 PHYSIOLOGY 



THE FUNCTIONS OF THE IRIS 



The iris is the forward prolongation of the pigment ed choroid coat. 

 It is covered anteriorly by a layer of epithelium continuous with Descemet's 

 epithelium, and behind by a thick layer of pigmented epithelium which is 

 prolonged forwards from the retina. It is composed of delicate connective 

 tissue, attached at its circumference to the fibres of the ligamentum pec- 

 tinatum, and contains two sets of unstriated muscular fibres. The one set, 

 the sphincter pupillce, is composed of fibres which run a circular course 

 around the margin of the pupil. The other set, the dilatator pupillce, forms 

 a flattened layer of radiating fibres, which lie close to the posterior surface and 

 extend from the attachment of the iris nearly to the rim of the pupil. 



The pigment in the iris and choroid serves the same purpos.e as the 

 blackened lining, which is supplied to every optical instrument, in preventing 

 reflection and dispersion of the incident light, and therefore preventing any 

 light falling on the retina except those rays which pass through the pupil 

 and refractive surfaces of the eye. The pigment in the iris has an additional 

 importance in that it enables this organ to act as a diaphragm. It not only 

 shields the retina, the sensory apparatus of the eye, from the effects of any 

 excess of illumination, but, by stopping out the rays of light passing through 

 the periphery of the lens, it diminishes spherical aberration and enables a 

 clear image of external objects to be formed on the back of the eyeball. The 

 diameter of the pupil is continually varying, according to the amount of light 

 falling into the eye and the condition of the mechanism of accommodation. 



Contraction of the pupil occurs under the following circumstances : 



(1) When light falls on the retina. This movement, which is known 

 as ' the light reflex,' is determined by a contraction of the sphincter pupillse, 

 together with a relaxation of the dilatator muscle. The contraction ensues 

 within a period of O04 to O05 sec. after the moment at which the light has 

 access to the retina, and attains its maximum within 0*1 sec. In man as well 

 as in other animals which have binocular vision, and in which there is a 

 partial decussation of the fibres of the optic nerves in the optic chiasma, 

 the reflex is bilateral, i.e. light falling into one eye causes simultaneous con- 

 traction of both pupils. In the higher animals this reaction of the pupil to 

 light demands the integrity of the nervous paths between the eye and the 

 brain ; but in many of the lower animals, e.g. in the frog and eel, the reflex 

 nervous mechanism is aided by a local sensibility of the iris to light. In these 

 animals the contraction of the pupil in response to illumination takes place 

 even in the excised eye, and seems to be determined by a direct stimulation 

 of the pigmented contractile fibres of the sphincter ptipillse by means of the 

 light. 



The effect of light on the pupil varies considerably according to the 

 condition of adaptation of the eye. The dilatation of the pupil is maximal 

 when the eye has been in the dark for some time and may amount then to 7-3 

 to 8 mm. In one experiment, on exposing the eye to a feeble light, e.g. 1-6 

 candles at a moderate distance, the pupil diminished in size to 6-3 mm. ; 



