DIOPTRICS OF THE EYE. 295 



was first explained satisfactorily by Helmholtz.* He attributed 

 it to the contraction of the ciliary muscle. This small muscle, 

 composed of plain muscle fibers, is found within the eyeball, lying 

 between the choroid and the sclerotic coat at the point at which the 

 sclerotic passes into the cornea and the choroid falls into the ciliary 

 processes. Some of its fibers take a more or less circular direction 

 around the eyeball, resembling thus a sphincter muscle, while others 

 take a radial direction in the,, plane of the meridians of the eye and 

 have their insertion in the choroid coat (Fig. 124). When this 

 muscle contracts the radial fibers especially will pull forward the 

 choroid coat. The effect of this change in the choroid is to loosen 

 the pull of the suspensory ligament (zonula Zinnii) on the lens and 

 this organ then bulges forward by its own elasticity. The theory 

 assumes that in a condition of rest the suspensory ligament, which 

 runs from the ciliary processes to the capsule of the lens, exerts a 



Ciliary Border 



process, of iris. Ciliary muscle. 



Pigment 

 epithelium 



Ora serrata. 



Fig. 124. Meridional section of eyeball after removal of sclerotic coat, cornea, and iris, 

 to show the position of the ciliary muscle. (Schultze.) 



tension upon the lens which keeps it flattened, particularly along 

 its anterior surface, since the ligament is attached more to this side. 

 When this tension is relieved indirectly by the contraction of the 

 ciliary muscle the elasticity of the lens, or rather of the capsule of 

 the lens, causes it to assume a more spherical shape along its anterior 

 surface, and the amount of this change is proportional to the 

 extent of contraction of the muscle. Other theories have been 

 proposed to explain the way in which the contraction of the ciliary 



* Helmholtz, "Handbuch der physiologischen Optik," second edition. 

 1896. 



