vi DIOPTEIC MECHANISM OF THE EYE 297 



among which that of Helmhnlt/ is almost universally accepted by 

 physiologists and ophthalmologists. 



The lens, lie says, is an clastic body which is kept radially 

 extended during the relaxation, of the internal eye-muscles, and 

 is somewhat compressed by the traction of the /onule adhering 

 to its 1 (order. During contraction the tension of the zonule and 

 its peripheral traction on the lens cease, and the lens then con- 

 tracts in the direction of its transverse diameter, and its axis 

 lengthens. This necessarily increases the curvature of its two 

 surfaces. 



Mannhardt (1850) and Schon (1885) attempted to oppose this 

 theory of distension or relaxation by the hypothesis of increased 

 tension of the zonule. According to these authors it is more 

 particularly the circular fibres and in a less degree the internal 

 radial fibres of the ciliary muscle that contract in accommodation. 

 In consequence of this contraction the ciliary processes are pulled 

 inwards and backwards, and the anterior ligaments of the zonule 

 are stretched and drawn back, causing increased curvature of the 

 lens, diminished hydrostatic pressure in the anterior chamber, and 

 increased pressure in the vitreous body. According to Tscherning 

 (1897), on the contrary, the contraction of the deep layer of the 

 ciliary muscle must draw the zonule backwards and outwards ; at 

 the same time the superficial layers of the ciliary muscle pull the 

 choroid forward and prevent the retraction of the lens. This 

 causes the peripheral zone of the lens to flatten, and its central 

 and denser portion becomes more concentrated. 



The later observations of Hess have established the arguments 

 of Helmholtz. The appearance of lateral oscillations in the lens 

 on forced accommodation, and the fact that after iridectomy the 

 ciliary body is pushed forward during Accommodation, are con- 

 clusive proofs of the earlier theory, while they contradict that of 

 Schon and Tscherning. 



Fig. 135 shows in a diagram the total mechanical effects wliich 

 result from the contraction of the different parts of the ciliary 

 muscle, according to the exhaustive observations of Hess. 



The ciliary muscle is innervated by the oculo-motor nerve. The 

 libres that regulate accommodation issue from the anterior mesial 

 nucleus of this nerve. On electrical stimulation of this nucleus 

 Hensen and Volckers (1878) obtained accommodation in dogs. 



The fibres of the oculo-motor that serve accommodation 

 terminate in the cells of the ciliary ganglion ; from this ganglion 

 other fibres are given off to form the short ciliary nerves, which 

 penetrate between the solera and choroid to reach the ciliary 

 muscle. Electrical stimulation of the separate short ciliary nerves 

 produces bulging of the choroid and displacement of the lens for 

 about 0-5 mm. (Hensen and Volckers). After poisoning with 

 nicotine, artificial stimulation of the trunks of the oculo-motor 



