128 ESSENTIALS OF PHYSIOLOGY. 



prism, dispersing the coloured rays which are combined in white light. 

 The rays at the violet end of the spectrum are of short wave-length 

 and are more refrangible, and therefore come to a focus sooner than 

 the rays of greater wave-length towards the red end of the spectrum. 

 Consequently the images formed on the retina are surrounded by violet 

 and red halos ; but these do not arouse any sensation, for two reasons : 

 (1) because the rays of medium refrangibility, which are brought to a 

 focus on the retina, are the most luminous, and the effect of the 

 stimulation excited by these is to depress the sensitivity of the 

 .adjacent parts of the retina by contrast, and (2) because the visual 

 apparatus is relatively insensitive to the rays at the extreme ends of 

 the spectrum. 



THE FUNCTIONS OF THE IRIS. 



The pupil varies in size with the degree of light entering the eye 

 and with other conditions, becoming smaller when the sphincter pupillee 

 contracts and wider on contraction of the dilator pupillse. We have 

 seen that the pupil contracts with accommodation, and that the result 

 is improved definition of the image on the retina. The improvement 

 is due to the cutting off of the peripheral rays, with the consequent 

 correction of spherical aberration. * Secondly, the iris regulates the 

 amount of light entering the eye, and so protects the retina from over- 

 stimulation. If the intensity of the light is gradually increased, the 

 pupil does not contract, but if the increase is sudden, the pupil becomes 

 smaller, and afterwards slowly returns to its former size as the retina 

 becomes adapted to the increased stimulus. On the other hand, in a 

 person in a dark room the pupils are widely dilated and remain in this 

 -condition until the eyes are once more exposed to light. The alteration 

 of the pupil with varying degrees of light is due in mammals to a 

 reflex nervous mechanism, the optic nerve conveying the afferent 

 impulses, and the third nerve and sympathetic fibres the efferent 

 impulses, to the sphincter and dilator of the pupil respectively. The 

 fibres to the sphincter travel by the ciliary ganglion and short ciliary 

 nerves, and stimulation of these in any part of their course is followed 

 by constriction of the pupil, while section of the third nerve is followed 

 by dilatation. The dilator fibres emerge from the spinal cord by the 

 first two thoracic anterior roots, and run up in the cervical sympathetic 

 to the superior cervical ganglion, from which post-ganglionic fibres run 

 along the internal carotid artery to the Gasserian ganglion, where they 

 join the ophthalmic division of the fifth nerve and travel to the dilator 

 fibres of the iris by the nasal branch and the long ciliary nerves 

 (fig. 40). Section of the cervical sympathetic nerve is followed by con- 



