106 



BIOPHYSICAL CHARACTERISTICS OF THE EYE 



nodal points (iV, N') lie at 7.08 and 7.33 mm, respectively; hence they 

 are nearly coincident with the posterior pole of the lens. 



Mechanism of Accommodation 



According to the Helmholtzian theory, the ciliary muscles (Fig. Ill— 5) 

 adjust the lens for near vision by removing the tension from the periph- 

 eral suspensory ligaments S. L. from which the lens is suspended, and 

 thus allow the lens capsule to expand as the result of its internal hydro- 

 static pressure. 



Lens 



Fig. Ill— 5. A diagrammatic representation of the mechanical interactions of 

 ciliary muscle L, suspensory ligaments S.L., and lens. If L contracts, the choroid is 

 stretched, b and c move down, removing tension from S.L., the suspensory ligaments 

 of the lens. The vector diagram is supposed to show how the external forces act on 

 the lense. S, canal of Schlemm; s.p., sphincter pupillae, circular muscle of the iris; 

 c.f., circular fibers of the ciliary muscle; L, longitudinal fibers of the ciliary muscle; 

 c, ciliary processes; O, ora serrata, end of retina. 



This change in shape of the lens is brought about indirectly through 

 the contraction of the ciliary muscle. The ciliary muscle consists of 

 two separate sets of unstriated muscle fibers : the more superficial set of 

 longitudinal fibers L, and the deeper set of bundles of circular fibers c.j. 

 The former originate at the sclerocorneal junction near S and are 

 attached to the anterior part of the choroid coat behind the ciliary proc- 

 esses below b. When these muscles contract they draw the choroid 



