SENSE OF VISION. 749 



ward, the pupil dilates under the influence of the dilatator papilla, which is 

 innervated by the Sympathetic nerve, and the tips of the ciliary processes 

 are approximated to the margin of the lens. The main instrument in effect- 

 ing these changes in the Eye appears to be the Ciliary Muscle, the structure 

 and attachments of which were first clearly described by Mr. Bowman. It 

 consists of unstriped muscular fibre, partly arranged in a circular manner, 

 the innermost fibres running parallel to the margin of the Cornea (d, Fig. 

 262), and partly disposed radially (a), the fibres of the latter portion of the 

 muscle appearing to be connected at their origin with the posterior elastic 

 lamina of the Cornea, and externally or posteriorly being partly inserted 

 into the Iris, forming the pillars of the Iris (Ligamentum pectinatum iridis), 

 partly into the Sclerotic bounding the canal of Schlemm (., Fig. 261 ), and 

 partly and chiefly into that part of the outer surface of the choroid tunic, 

 which corresponds to the ciliary processes. The mode and effects of the 



FIG. 262. 



V 



Section of the Ciliary Region of the Eye in Man. a. Meridional muscular fasciculi of the imiseuhis 

 ciliaris. 6. Deeper seated radiating fasciculi, c, c, c. Annular plexus, d. Annular muscle of Miiller. 

 /. Muscular lamina on the posterior surface of iris. g. Muscular plexus at the ciliary border of the 

 Iris. e. Annular tendon of the musculus ciliaris. h. Ligarneutum pectinatum. 



contraction of this muscle have not been determined with perfect certainty ; 

 some, as Briicke, believing the anterior attachment to be the origin, or fixed 

 point, towards which in contraction the posterior extremity is drawn ; others, 

 as Bonders, that the posterior border is the true origin ; and others again, as 

 Helmholtz, considering that both extremities are movable. According to 

 Cramer, Bonders, H. Miiller, and others, the lens, when adapted for viewing 

 infinitely-distant objects, is at rest, and it is only when near objects are 

 looked at that a change is effected in the figure and position of the lens ; 

 Cramer attributing this effect especially to pressure exerted by the iris, and 

 H. Miiller to the ciliary muscle. Helmholtz, Arlt, and Jiiger consider that 

 the lens is flattened in viewing distant objects, through the traction exerted 

 by an elastic membranous zouula attached to its edge. When the ciliary 

 muscle contracts, which occurs in viewing near objects, the posterior border 

 of the zonula is drawn forward, and its tension, and therefore the flattening 

 power it exerts upon the lens, is diminished. A third supposition has been 

 advanced by Henke and Langeubeck 1 to the effect that a muscular effort is 

 required in arranging the Eye for viewing both near and distant objects, the 

 former being accomplished through the contraction of the circular fibres, the 

 latter through that of the radial fibres ;' 2 but a strong argument against this 



1 Henle and Meissner's Bericht, 1860, p. 561. 



2 An opinion to which Gi'afe seems also to have been led from a comparison of the 

 effects of belladonna and opium upon the iris and upon the ciliary muscle (as indi- 

 cated by the power of accommodation) respectively. See also Warlomont, Annales 

 d'Oculistique, t. 73, p. 195. 



