304 



THE SPECIAL SENSES. 



commonly the case, it is due to an irregularity in the curvature 

 along some one meridian, such as may be produced by a scar upon 

 the cornea. This condition may be produced from a variety of 

 causes affecting either the cornea or the lens, and practically it 

 can not be corrected by the use of lenses. As Helmholtz has shown, 



a small degree of irregular 

 astigmatism is present nor- 

 mally, owing to a certain 

 asymmetry in the curvature 

 of the lens. This defect is 

 made apparent in the visual 

 sensations caused by a point 

 of light, such as is furnished, 

 for instance, by a fixed star. 

 The retinal image in these 

 cases, instead of being a sym- 

 metrical point, is a radiate 

 figure the exact form of 

 which may vary in different 

 eyes. For this reason the 

 fixed stars give us the well- 

 known star-shaped image 



Fig. 127. Astigmatic chart. 



instead of a clearly defined 



point. 



Innervation and Physiological Control of the Ciliary Muscle 

 and the Muscles of the Iris. From an optical point of view the 

 iris plays the part of a diaphragm. It is, moreover, an adjustable 

 diaphragm the aperture of which that is, the size of the pupil 

 is varied reflexly according to the conditions of illumination. Its 

 adjustments are made possible by the fact that it contains within 

 its substance two bands of muscular tissue, one, the sphincter 

 muscle, forming a circular ring whose contraction diminishes the 

 aperture of the pupil, and the other a dilator muscle whose contrac- 

 tion widens the pupil. Each of these muscles possesses its own 

 nerve fibers that arise ultimately from the brain, and through these 

 fibers reflex movements of great delicacy are effected. The sphinc- 

 ter pupillae is a well-defined band of plain muscle whose width 

 varies, according to the state of contraction, from 0.6 to 1.2 mms.; 

 it forms a ring lying just on the margin of the pupil, and it is im- 

 bedded in the stroma of the iris. The histological differentiation 

 of the dilator pupillse is much less distinct. For a long time its 

 existence was the subject of controversy, but it is now conceded 

 that such a muscle is present in the form of a layer of elongated 

 spindle-like cells which lie close to the pigment layer of the iris and 



