Eyes of Mollusca and Arthropods. 57 \ 



(which iu Pecicii Jacohaeus and Pecten opercularis projects but little 

 above the level of the h-is) , and the size of the pnpil, may be regulat- 

 ed, aecordiug to the couditions tobe fultilled, by means of contractile 

 fibres. By the simultaueous contraetion of the ciliar is (PI. 29, figs. 18 

 and 19 s. b.) and the superficial circular and radiating- fibres of the lens, 

 the edges of the cornea and iris will be drawn inwards, and the convex- 

 ity of the former and of tlie outer surface of the lens increased, while the 

 opening of the pupil may be diminished to almost half its previous dia- 

 raeter. In the living condition, all stages of contraetion may be ob- 

 served. Streng irritants cause contraetion, and an increased convexity of 

 the cornea. If the mantle is treated with weak chromic acid, the muscles 

 relax, the cornea becomes nearly fiat, and the pupil widely extended. 

 Contraetion of the longitudinal muscles, extending along the inner wall 

 of the iris and attached to the suspensory ligament (PI. 29, 

 fig. 19 s. b.), would cause a flattening of the outer surface of the lens, 

 accompanied by a bodily movement of the latter toward the retina. We 

 now see the object of the peculiar layer of vertical nerve fibres ; for 

 without this, any movement of the lens, attached to the septal mem- 

 brane, would necessitate a disturbance of, or injury to the retinal cells. 

 But with the present arrangement the lens, accompanied by the septal 

 membrane, may be elevated or depressed, without contact with the 

 retina, simply by the flexion of the nerves of the fibrous layer. The 

 latter was considered by Hensen to be artificially formed by shrink- 

 age, and the consequent drawing out of the nerve fibres from the 

 septal membrane. Cakkière has described it as forming a layer of small 

 columnar cells. That both of these Statements are incorrect, and that 

 the condition represented in figs. 10 and 19 is a natural one, may 

 be Seen by examining this layer, in the living condition, through the 

 choroid fissure. It is very probable that the inward movement of the 

 lens is accomplished by the contraetion of the ciliaris and the 

 muscles attached to the suspensory ligament. This movement is 

 facilitated by the contraetion of the circular fibres on the peri- 

 phery of the septal membrane, so that the thick, structureless, 

 central portion, upon which the lens rests, will fall by its own weight, 

 or by the pressure of the lens. But how is the elevation of the latter 

 accomplished? It might be done by the contraetion of the relaxed, septal 

 membrane; but that is only provided with circular fibres, which could 

 not produce any such effect. Neither could the lens be raised by the con- 

 traetion of any of its own muscles, or of any attached to it. It is pro- 

 ducedbythetendency oftheelastie septal membrane to 



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