THE IRIS. 843 



of the iris, is excised; and, further, that, after excision of this ganglion, atropin 

 has less effect upon the pupil of the same side. The increased dilatation of the 

 pupil by irritation of the sympathetic after instillation of atropin is probably 

 also the result of diminished injection of the vessels of the iris. If an animal 

 whose pupil is dilated by atropin be bled to death quickly, the pupil contracts 

 on account of the irritation of the oculomotor center by the anemia. The dila- 

 tation of the pupil in cases of trigeminal neuralgia must be referred partly to 

 irritation of the dilator fibers, and partly to irritation of the vasomotor fibers 

 of the iris. 



5. Contraction of the pupil occurs as an associated movement during accom- 

 modation for near vision, further, as a result of strong effort to close the lids, 

 and in rotation of the eyeball inward,, which is the case during sleep. Conversely, 

 intense movement of the iris, caused by variations in the brightness of dazzling 

 lights, for example of electric light, produces disturbing associated movements 

 of the ciliary muscles. In connection with certain movements excited in the 

 medulla oblongata (forced breathing, chewing, swallowing, vomiting) dilatation 

 of the pupil occurs as a kind of associated movement. 



Direct stimulation of the corneal limbus causes dilatation of the pupil. In 

 fact partial dilatation may be produced by direct irritation of a circumscribed 

 portion of the margin of the iris, by contraction of the dilator fibers, although 

 also the sphincter contracts at the same time. 



If a flame be placed in a dark room on one side of an eye directed straight 

 ahead, and attention is suddenly directed to the flame, without changing the 

 direction of vision, the pupil contracts. This movement is known as the "cortical 

 reflex." Other things being equal, an analogous dilatation of the pupil also 

 takes place; one may observe variations in the size of the pupil from the mere 

 conception of light or darkness, even in the blind. Bechterew saw a case of uni- 

 lateral voluntary dilatation of the pupil. 



As to the action of poisons on the iris ignorance still prevails. The mydriatics 

 cause dilatation: Atropin, homatropin, duboisin, scopolamin, daturin, hyoscyamin, 

 hyoscin, probably through paralysis of the oculomotor chiefly. They must also 

 stimulate the dilating fibers at the same time, for in the presence of complete 

 oculomotor palsy the moderately dilated pupil is still further dilated by atropin. 

 Minimal doses of atropin cause contraction of the pupil by stimulation of the 

 pupil-contracting fibers. Excessive doses cause moderate dilatation as the 

 result of paralysis of both the dilating and contracting fibers. Atropin acts 

 even after destruction of the cijiary ganglion, in fact on the enucleated eye. 



For the action of the constrictors, or myotics: physostigmin (or eserin, the 

 alkaloid of physostigma) , nicotin, pilocarpin, muscarin, and morphin, some 

 investigators assume a stimulation of the oculomotor, others a paralysis of the 

 sympathetic. As these drugs cause contraction of the ciliary muscle Griinhagen 

 supposes an analogous action upon the sphincter. In all probability they para- 

 lyze the dilator fibers, and stimulate the oculomotor fibers at the same time. 



Intravenous injection of suprarenal extract causes all signs of irritation of 

 the cervical sympathetic in the eye. 



If one pupil is contracted or dilated by these drugs, the other pupil is con- 

 versely dilated or contracted on account of the variation in the amount of light 

 that enters the eye into which the drug has been introduced. 



The Anesthetics. Chloroform, in the excitation-stage of narcosis (beginning 

 of unconsciousness), stimulates the center for the dilatation of the pupil. Later, 

 this center is paralyzed (so that no dilatation occurs on the application of external 

 stimuli). Then, the contracting center is 'stimulated (the pupil becoming reduced 

 to the size of a pinhead) , and finally (with danger of death) this center becomes 

 paralyzed, and the pupil dilates. 



The movements of the iris are always accompanied by variations in the intra- 

 ocular pressure. Dilatation of the pupil increases, contraction of the pupil 

 diminishes the intraocular pressure. Irritation of the sympathetic increases, 

 section diminishes, the pressure. Instillation of atropin, after a short temporary 

 lowering of the pressure, produces an increase. Eserin, after a primary increase, 

 causes diminution of the pressure. 



According to Hocker, atropin decreases the pressure ; eserin increases it pri- 

 marily and then decreases it on the appearance of myosis. 



Reflex dilatation of the iris occurs slightly later than reflex contraction: 

 respectively 0.5 and 0.3 second after the light-stimulus. A certain period of 

 time always elapses before the size of the pupil adapts itself to the amount of 



