812 THE SENSE OF SIGHT 



gated, spindle-shaped cells close to the pigment layer. ^ The blood- 

 supply of the iris is derived from the long and anterior ciliary arteries, 

 and its nerve supply from the long and short ciliary nerves. The 

 origin and function of these nerves will be more fully described in a 

 later paragraph. 



The Function of the Iris. — The action of the iris may be compared 

 to that of the adjustable diaphragm of an ordinary photographic 

 camera. As such it possesses two functions, namely, to: 



(a) Vary to size of the bundle of light entering the vitreous cavity, (1) during far 

 and near vision or accommodation, and (2) during the alterations in the intensity 

 of the light. 



(6) Direct the rays of light through the center of the lens which is its most per- 

 fectly refracting part. Thus, by excluding the margins of lens, it prevents the 

 occurrence of spherical and chromatic aberration. 



It should be evident from the preceding discussion pertaining to 

 the structure of the iris, that the contraction of its circular muscle 

 fibers decreases the size of the pupil, while the contraction of its radial 

 fibers increases it. Thus, we find at times that the margin of the 

 iris is drawn almost completely over the lens, lessening the diameter 

 of the pupil to less than 1 mm., and, at other times, that it is pulled 

 outward until this aperture measures as much as 8 mm. across. The 

 former change constitutes pupillar constriction, and the latter, pupillar 

 dilatation. Obviously, these changes either diminish or increase the 

 number of the light rays entering thie vitreous chamber. A diminution 

 in their number is mad6 necessary (a) when the intensity of the light 

 is great, and (b) when the eye is adjusted for a near object. Con- 

 versely, an increase in their number is required (a) in low intensities of 

 light, and (6) when the object accommodated for is situated far away 

 from the eye. Furthermore, inasmuch as these changes are effected 

 as a result of reflex stimulation, we commonly speak of them as the 

 light and accommodation reflexes. 



The Light Reflex. — If a person is made to look alternately from a 

 partially darkened surface into a light of moderate intensity, it will 

 be observed that the pupil becomes small whenever the eye is more 

 fully illuminated. An intense hght, in fact, decre.ses its size to almost 

 that of the point of a pin. It is true, however, that this change in the 

 illumination must be effected rather rapidly, otherwise a decided 

 alteration in the size of the pupil will not be produced. ^ Moreover, 

 if the constriction has been continued for a longer time than 3 or 4 

 minutes, its size gradually increases, owing to an adaptation and 

 fatigue of the constrictor mechanism. Obviously, the purpose of an- 

 enlarged pupil is to augment the receptive power of the retina by 

 permitting as many rays as possible to strike it, while a constricted 

 pupil serves to protect the retinal elements against an undue and in- 

 jurious degree of stimulation. 



1 Grumert, Arch, fiir Augenheilkunde, xxxvi, 1898. 



2 Garten, Pfltiger's Archiv, Ixviii, 1897, 68. 



