446 SPECIAL PHYSIOLOGY. 



pendent on the focal distance of the eyes, and in individuals possess- 

 ing ordinary visual powers, present but slight differences. 



In some persons, the refractive power of the horizontal and vertical 

 meridians of the eye is unequal. This condition, which is not uncom- 

 mon, is known as astigmatism. It is due to a difference in the degree 

 of convexity of the cornea or crystalline lens, or of both these parts, 

 in the horizontal and vertical directions, so that corresponding rays 

 passing into the eye, instead of converging to one identical point, 

 meet at two different foci. By means of cylindrical glasses, this im- 

 perfection can be corrected. 



The dilatation and contraction of the pupil, which result from varia- 

 tions of light, are purely reflex phenomena. The sensory fibres of the 

 iris, as well as its vasi-motor fibres, are derived from the fifth cranial 

 nerve ; irritation of either the first or second divisions of this nerve, 

 causes the pupil to contract on the same side. 



The direct effects of heat and light on the iris, in dead animals, 

 have been noticed by Brown-Se'quard. If the eye of a rabbit, or 

 other animal, be subjected, shortly after death, to a sudden elevation 

 of temperature of from 50 to 60, the pupil, if previously contracted, 

 dilates, or, if dilated, it contracts ; these effects are explained by sup- 

 posing that the heat acts more powerfully on those muscular fibres 

 which had previously been at rest. Light causes contraction of the 

 pupil in Mammalia and Birds, for a short time after death ; but in 

 eels and frogs, the iris may be so excited even sixteen days after ! 

 Yellow light seems to act the most powerfully. This sort of contrac- 

 tion can be induced after removal of the posterior half of the eyeball, 

 so that it cannot be referred to reflex action. 



During life, the diameter of the pupil diminishes with increasing 

 light, the amount of contraction being proportionate to the strength 

 and duration of the luminous impression. A direct light acts more 

 powerfully than a slanting light, the sides of the retina being appa- 

 rently less' excitable. When the two pupils are under the influence 

 of different degrees of light, they are still usually of equal size, unless 

 the difference of luminosity be very great. If one eye only be acted 

 upon by light, both pupils contract, the one not exposed to the stimu- 

 lus rather less than the other. Contraction of the pupil takes place 

 more rapidly than dilatation; and it has further been noticed that the 

 movements of the iris are quicker than those of other parts composed 

 of unstriped muscular fibres. Excessive dilatation of the pupil is 

 named mydriasis, and excessive contraction, myosis. 



The chief object of these changes in the size of the pupil, is to regu- 

 late the quantity of light admitted to the eye, and to protect it against 

 too dazzling luminous rays; they also, in this way, determine the 

 brightness of the retinal images, which become brighter as the pupil di- 

 lates, though they may be then less defined or distinct. 



As already mentioned, during near vision a consentaneous contrac- 

 tion of the pupil occurs. It has been noticed, indeed, that the pupils 

 also contract when the eyes are made to converge, and, as this con- 

 vergence always accompanies the act of looking at near objects after 

 distant ones, this likewise aids in inducing the contraction of the pupils 





