THE SENSE OF VISION. 



323 



in mind that the iris, seen in this way, does not appear in its natural size and 

 position, but somewhat enlarged and bulged forward by the magnifying effect 

 of the cornea and the aqueous humor. The changes in one's own pupil may 

 be readily observed by noticing the varying size of the circle of light thrown 

 upon the retina when the eye is illuminated by a point of light held at the 

 anterior focus, as in the method above described for the study of intraocular 

 images. 



The muscles of the iris are, except in birds, of the unstriped variety, and 

 are arranged concentrically around the pupil. Kadiating fibres are also recog- 

 nized by many observers, though their existence has been called in question 

 by others. The circular or constricting muscles of the iris are under the con- 

 trol of the third pair of cranial nerves, 

 and are normally brought into activity 

 in consequence of light falling upon 

 the retina. This is a reflex phenom- 

 enon, the optic nerve being the affer- 

 ent, and the third pair, the ciliary 

 ganglion, and the short ciliary nerves 

 the efferent, channel, as indicated in 

 Figure 142. This reflex is in man 

 and many of the higher animals bi- 

 lateral — i. e. light falling upon one 

 retina will cause a contraction of both 

 pupils. This may readily be observed 

 in one's own eye when focally illumi- 

 nated in the manner above described. 

 Opening the other eye will, under 

 these conditions, cause a diminution, 

 and closing it an increase, in the size 

 of the circle of light. This bilateral 

 character is found to be dependent 

 upon the nature of the decussation of 

 the optic nerves, for in animals in 

 which the crossing is complete the 

 reflex is confined to the illuminated 

 eye. The arrangement of the fibres 



Course of constrictor nerve-fibres 

 " dilator " 



Fig. 142. — Diagrammatic representation of the 



nerves governing the pupil (after Foster) : U, optic 



nerve: I. g, ciliary ganglion; r.b, i is Bbort rout from 



III, motor-oculi nerve; sym, its sympathetic root; r.i, 



in the optic commissure is in general its i.mg mot from r. o phthalmo-nasal branch of oph- 



. , • i i • • n , thalmic division of fifth nerve; $.c, short ciliary 



associated with the position of the m , n , f ciliary nerves. 



eyes in the head. When the eyes 



are so placed that they can both be directed to the same object, as in man 



and many of the higher animals, the fibres of each optic nerve arc usually 



found to be distributed to both optic, tract-, while in animal- whose eves 



are in opposite sides of the head there is complete crossing of the optic nerves. 



Hence it may be said that animals having binocular vision have in general 



a bilateral pupillary reflex. The rule is. however, not without exceptions, 



for owls, though their visual axes are parallel, have, like other birds, a com- 



