The iris is usually said to consist of two muscles, the sphincter, which has circular fibres and contracts the pupil, and the dilator, which has radial fibres and dilates the pupil. All observers are agreed regarding the sphincter muscle of the eyes, but some deny the existence of the dilator muscle. In the following description, however, I shall take the view which is usually accepted.2

The sphincter receives its motor nervous supply from the third nerve, and the dilator from the cervical sympathetic. The nervous centre for the contraction of the pupil probably lies in the corpora quadrigemina; the nerve-centre for the dilatation of the pupil lies in the medulla oblongata, but there seems to be another dilating centre, situated in the floor of the front part of the aqueduct of Sylvius.3 The contracting nerves are contained in the third nerve, and pass to the ciliary ganglion, and thence to the eye. Along with them motor fibres pass also to the ciliary muscle. This muscle when contracted lessens the tension of the suspensory ligament on the lens, allowing the latter to become more spherical, and thus accommodating the eye for near objects. Such accommodation and contraction of the pupil generally accompany one another. The arrangement of the nerves of the eye is very diagrammatically shown in Fig. 74. A few of the dilating fibres are contained in the fifth nerve, but most of them pass, down the spinal cord to the cilio-spinal region in the lower cervical and upper dorsal part of the cord, and thence through the second dorsal nerve in monkeys and probably in man, or through the inferior cervical and superior dorsal nerves in the rabbit, into the cervical sympathetic, in which they again ascend to the eye.

1 Maynard, Virchow's Archiv, vol. lxxxix. p. 258.

2 At present it is generally assumed that muscular fibres, either voluntary or involuntary, contract only in the direction of their length. If we suppose that they can contract either in the direction of their length or their width, the movements of the iris might be more readily explained. At present we assume the presence of a dilator muscle, which is almost certainly absent in many animals, in order to explain phenomena which might be explained just as readily by the supposition that the muscular fibres which are present can contract in two directions (see p. 117).

3 Foster's Physiology, 4th ed.

Fig. 74.   Diagram to show the nervous supply of the eye. a, nerves to the ciliary muscle regulating accommodation; b, nerves to the contracting fibres, and c, nerves to the dilating fibres of the iris; d, vaso motor nerves to the vessels of the eye.

Fig. 74. - Diagram to show the nervous supply of the eye. a, nerves to the ciliary muscle regulating accommodation; b, nerves to the contracting fibres, and c, nerves to the dilating fibres of the iris; d, vaso-motor nerves to the vessels of the eye. The iris is put apparently behind instead of in front of the lens for convenience in showing the passage of nerves to it.

Along with the dilating fibres others pass to supply the orbital muscle at the back of the orbit, which causes protrusion of the eyeball, as already mentioned. There are also other fibres from the sympathetic (vaso-motor) which supply the muscular coats of the arteries of the ciliary vessels.

The dilating centre may be stimulated directly by venous blood circulating in it. In consequence of this the pupils usually dilate much when the respiration is imperfect, as during dyspnoea; but when the asphyxia becomes complete the centre again becomes paralysed and the size of the pupil diminishes. It may be stimulated reflexly by irritation of sensory nerves, so that dilatation of the pupil has been used as an indication of sensation in animals paralysed by curare. It seems to be readily stimulated by irritation of the genital organs. This is probably the reason why dilatation of the pupil frequently occurs in persons suffering from irritation of the genital organs. It is probably also readily stimulated by irritation of the intestinal canal, and such irritation may be the cause of dilatation of the pupil in children suffering from worms, and in cases of poisoning by drugs which irritate the gastro-intestinal canal, like aconite.

The drugs which act upon the iris are divided into two classes: Mydriatics which dilate, and Myotics which contract the pupil. The most important of these are such drugs as have a local action on the eye, and they alone are used in ophthalmic medicine. They are indicated in the following list by an *.