DIOPTRIC MECHANISMS. 751 



still to possess some accommodation, we must suppose that no real accommo- 

 dation took place, but that the pupil contracted when a near object was 

 looked at, and so assisted in making vision more distinct. 



626. This increase of the convexity of the lens has been supposed to 

 be due to a compression of the circumference of the lens by a contraction of 

 the iris ; but this is disproved by the fact that accommodation may take 

 place in eyes from which the iris is congenitally absent. It has also been 

 attributed to vasomotor changes, to increased fulness of the vessels of the iris 

 or ciliary processes surrounding the lens ; but this also is disproved by the 

 fact that accommodation may be effected after death, in an eye which is 

 practically bloodless, by stimulating the ciliary ganglion or ciliary nerves 

 with an interrupted current or by other means. The real nature of the 

 mechanism seems to be as follows. 



627. The lens when examined after removed from the eye is found to 

 be a body of considerable elasticity. When the curvature of the anterior 

 surface of the lens is determined, as may be done by appropriate means, in 

 its natural position in the eye at rest, and then again determined after the 

 lens has been removed from the eye, the anterior surface is found to be more 

 convex in the latter than in the former case. There seems to be in the eye 

 in its natural condition, some agency at work keeping the anterior surface 

 of the lens somewhat flattened. The suspensory ligament, attached to the 

 choroid and ciliary processes behind, and passing over the front of the lens, is 

 just such a structure as would produce this effect. In the natural position of 

 the choroid this ligament is tense, and tends to flatten the front of the lens. 

 When the choroid is pulled forward, the ligament becomes slack and the 

 lens bulges out forward. Further, the ciliary muscle attached on the one 

 hand to a fairly fixed region, the junction of the sclerotic and cornea, and 

 on the other to the looser and more movable choroid, would naturally, when 

 thrown into contraction, pull forward the choroid and so slacken the suspen- 

 sory ligament, and hence permit the elastic lens to bulge out forward. And 

 we have experimental evidence, carried out on lower animals, that stimula- 

 tion of the ciliary ganglion or of its so-called radix brevis does lead, on the 

 one hand, to a contraction of the ciliary muscle and pulling forward of the 

 choroid, and, on the other hand, to an increased curvature of the anterior 

 surface of the lens. Hence we may conclude that accommodation for near 

 objects consists essentially in a contraction of the ciliary muscle, which, by 

 pulling forward the choroid coat and the ciliary process, slackens the sus- 

 pensory ligament, and allows the lens to bulge forward by virtue of its elas- 

 ticity, and so to increase the convexity of its anterior surface [Figs. 170 and 

 171]. 



Accommodation is in most cases a voluntary act ; since, however, the 

 change in the lens is always accompanied by movements in the iris, it will 

 be convenient to consider the latter before we discuss the nervous mechan- 

 ism of the whole act. 



628. Movements of the pupil. Though by making the efforts required 

 for accommodation we can at pleasure contract or dilate the pupil, it is not 

 in our power to bring the will to act directly on the iris by itself. This 

 fact alone indicates that the nervous mechanism of the pupil is of a peculiar 

 character, and such indeed we find it to be. The pupil is contracted (1) 

 when the retina (or optic nerve) is stimulated, and when light falls on the 

 retina, the brighter the light the greater being the contraction, (2) when we 

 accommodate for near objects. The pupil is also contracted when the eye- 

 ball is turned inward, when the aqueous humor is deficient, in the early 

 stages of poisoning by chloroform, alcohol, etc. ; in nearly all stages of 

 poisoning by morphine, physostigmine, and some other drugs ; and in deep 



