206 HAND-BOOK OF PHYSIOLOGY. 



of chloroform and alcohol poisoning; (6) on division of the cervical sym- 

 pathetic or stimulation of the third nerve. Dilatation of the pupil 

 occurs (1) in a dim light; (2) when the eye isfocussed for distant objects; 

 (3) on the local application of atropine and its allied alkaloids; (4) on the 

 internal administration of atropine and its allies; (5) in the later stages 

 of poisoning by chloroform, opium, and other drugs; (6) on paralysis 

 of the third nerve; (7) on stimulation of the cervical sympathetic, or of 

 its centre in the floor of the front of the aqueduct of Sylvius. The con- 

 traction of the pupil appears to be under the control of a centre in the 

 corpora quadrigemina; and this is reflexly stimulated by a bright light, 

 and the dilatation when the reflex centre is not in action is due to the 

 more powerful sympathetic action; but in addition, it appears that both 

 contraction and dilatation may be produced by a local mechanism, upon 

 which certain drugs can act, which is independent of and probably often 

 antagonistic to the action of the central apparatus of the third and sym- 

 pathetic nerves. The action of the fifth nerve upon the pupil is not well 

 understood, but its apparent effect in producing dilatation is due to the 

 mixture of sympathetic fibres with its nasal branch. The sympathetic in- 

 fluence upon the radiating fibres is believed to be conveyed, not by the 

 long ciliary branches of that nerve, but by the short ciliary branches from 

 the ophthalmic ganglion. 



The close sympathy subsisting between the two eyes is nowhere better 

 shown than by the condition of the pupil. If one eye be shaded by the 

 hand its pupil will of course dilate; but the pupil of the other eye will 

 also dilate, though it is unshaded. 



Ciliary Muscle. The ciliary muscle is composed of plain muscular 

 fibres, which form a narrow zone around the interior of the eyeball, near 

 the line of junction of the cornea with the sclerotic, and just behind the 

 outer border of the iris (Fig. 365). The outermost fibres of this muscle 

 are attached in front to the inner part of the sclerotic and cornea at their 

 line of junction, and diverging somewhat, are fixed to the ciliary pro- 

 cesses, and a small portion of the choroid immediately behind them. 

 The inner fibres immediately within the preceding, form a circular zone 

 around the interior of the eyeball, outside the ciliary processes. They 

 compose the ring formerly called the ciliary ligament. 



Accommodation of the Eye. The distinctness of the image 

 formed upon the retina, is mainly dependent on the rays emitted by each 

 luminous point of the object being brought to a perfect focus upon the 

 retina. If this focus occur at a point either in front of, or behind the 

 retina, indistinctness of visioif ensues, with the production of a halo. 

 The focal distance, i.e., the distance of the point at which the luminous 

 rays from a lens are collected, besides being regulated by the degree of 

 convexity and density of the lens, varies with the distance of the object 

 from the lens, being greater as this is shorter, and vice versd. Hence, 



