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ESSENTIALS OF PHYSIOLOGY. 



in accommodation, and also that it becomes more convex. The absence 

 of movement of the other images indicates that the cornea and posterior 

 surface of the lens remain stationary during accommodation. 



The Mechanism of Accommodation. The lens is an elastic structure, 

 and, as has already been stated, it is enclosed in a capsule which is 

 connected with the ciliary processes by the suspensory ligament. The 

 contents of the eyeball exert a pressure or tension on the coats of the 

 eye, amounting normally to the equivalent of 25 mm. of mercury. 

 In consequence of this tension the suspensory ligament exerts a pull 

 on the lens capsule, and the convexity of the anterior surface of the 

 lens is in this way diminished. When the lens is removed from the 

 eye it assumes a more convex shape in virtue of its elasticity. The 

 same change of shape takes place with the lens in position, when the 



FAR 



NEAR 



FIG. 37. Diagram showing mechanism of accommodation. 

 (After Helmholtz and Foster. ) 



a, suspensory ligament ; 6, ciliary muscle. 



tension of the suspensory ligament is diminished during accommodation 

 by the action of the ciliary muscle. Contraction of the radial fibres 

 of this muscle pulls forward the posterior part of the ciliary processes 

 with the attached suspensory ligament, and in this way the latter is 

 relaxed and the lens becomes more convex anteriorly in virtue of its 

 elasticity (fig. 37). The circular fibres of the ciliary muscle also take 

 part in accommodation, approximating the ciliary process to the lens 

 by their contraction. The effect of contraction of the ciliary muscle 

 may be demonstrated by two experiments : (1) by the action of eserine, 

 and (2) by the movement of a needle inserted into the ciliary processes. 

 (1) When eserine is instilled into the human eye, it causes the ciliary 

 muscle and sphincter of the iris to contract, and the slackening of the 

 suspensory ligament can be shown by oscillations of the lens which 

 take place when the head is quickly moved. (2) When the point of a 

 needle is inserted into the ciliary processes in an animal, and the 

 ciliary muscle is stimulated to contract, the end of the needle outside 



