THE SENSES. 595 



diaphragm, provided with a central aperture, the pupil, for the trans- 

 mission of light. It is composed of plain muscular fibres imbedded in 

 ordinary fibro-cellular or connective tissue. The muscular fibres have a 

 direction, for the most part, radiating from the circumference towards 

 the pupil; but as they approach the pupillary margin, they assume a cir- 

 cular direction, and at the very edge form a complete ring. By the con- 

 traction of the radiating fibres (dilator pupillae) the size of the pupil is 

 enlarged: by the contraction of the circular ones (sphincter pupillae), it 

 is diminished. The object effected by the movements of the iris, is the 

 regulation of the quantity of light transmitted to the retina. The pos- 

 terior surface of the iris is coated with a layer of dark pigment, so that 

 no rays of light can pass to the retina, except such as are admitted 

 through the aperture of the pupil. 



This iris is very richly supplied with nerves and blood-vessels. Its 

 circular muscular fibres are supplied by the third (by the short ciliary 

 branches of the ophthalmic ganglion), and its radiating fibres, by the 

 sympathetic and fifth cranial nerve (by the long ciliary branches of the 

 nasal nerve). 



Contraction of the pupil occurs under the following circumstances: 

 ( I) On exposure of the eye to a bright light; (2) when the eye is focussed 

 for near objects; (3) when the eyes converge to look at a near object; 



(4) on the local application of eserine (active principle of Calabar bean); 



(5) on the administration internally of opium, aconite, and in the early 

 stages of chloroform and alcohol poisoning; (6) on division of the cer- 

 vical sympathetic or stimulation of the third nerve. 



Dilatation of the pupil occurs (1) in a dim light; (2) when the eye is 

 focussed 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; (?) on stimulation of 

 the cervical sympathetic, or of its centre in the floor of the front of the 

 aqueduct of Sylvius. The contraction of the pupil appears to be under 

 the control of a centre in the medulla or on 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 sympathetic nerve. 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 sympa- 

 thetic fibres with its nasal branch. The sympathetic influence upon the 

 radiating fibres is believed to be conveyed not by the long ciliary 



