ACTION OF THE IKIS. 243 



Many experiments have been made to discover the nerve, which pre- 

 sides over the movements of the iris. These experiments have demon- 

 strated, that if, instead of directing a pencil of rays upon the iris, we 

 throw it on the retina, or through the retina on the choroid, contrac- 

 tion of the pupil is immediately induced. The movements of the iris 

 must, then, be to a certain extent under the influence of the optic as 

 an afferent nerve. It is found, indeed, that if the optic nerve be 

 divided on a living animal, the pupil becomes immovable and ex- 

 panded. Yet, that the motions of the iris are not solely influenced 

 by this nerve is evinced by the fact, that in many cases of complete 

 amaurosis of both eyes, there has been the freest dilatation and 

 contraction of the pupil; and also, that section of the nerve of the 

 fifth pair, which chiefly supplies the iris, equally induces immobility of 

 the pupil. The same effect is produced, according to Mr. Mayo, 1 by 

 dividing the third pair. If the trunk of that nerve be irritated, con- 

 traction of the pupil is seen to follow ; and, according to Desmoulins, 2 

 in the eagle, whose iris is extremely movable, the third is the only 

 nerve distributed to the organ. The general remark, made by M. 

 Broussais 3 on the organs that combine voluntary and involuntary func- 

 tions, has been considered applicable here ; that they will be found to 

 possess both cerebral and ganglionic nerves. Accordingly, M. Magen- 

 die 4 conjectures, that those of the ciliary nerves, which proceed from 

 the ophthalmic ganglion, preside over the dilatation of the pupil, or 

 are the nerves of involuntary action; and that those which arise from 

 the nasal branch of the fifth pair, preside over its contraction. We 

 might thus understand why, in apoplexy, epilepsy, &c., the pupil 

 should be immovably dilated. All volition and every cerebral pheno- 

 menon are abolished by the attack : the nerve of the fifth pair, there- 

 fore, loses its influence; and the iris is given up to the agency of the 

 ganglionic nerves or nerves of involuntary action proceeding from the 

 ophthalmic ganglion. 



On the whole, our notions regarding the motions of the iris, and 

 the nerves that preside over them, must be esteemed vague and unsatis- 

 factory : and the obscurity is not diminished by a remark of Bellingeri. 5 

 The iris, he observes, derives its nerves from the ophthalmic ganglion, 

 which is formed by the fifth in conjunction with the third pair ; and its 

 involuntary motions, he thinks, are regulated by the fifth pair. In 

 those instances, in which the motions of the iris have been found de- 

 pendent on the will, Bellingeri argues, that the ciliary nerves received 

 no branches from the fifth a fact, which has been proved by dissec- 

 tion, as well as by the circumstance, that in the parrot, owl, and the 

 ray genus among fishes in which the iris is under the will of the ani- 

 mal there is no ophthalmic ganglion. 



1 Commentaries, P. ii. p. 5, arid Outlines of Human Physiology, &c., 4th edit., p. 287, 

 Lond., 1837. 



3 Anatom. des System.lNerveux, Paris, 1825. 



3 Traite de Physiologic appliquee a la Pathologic, translated by Drs. Bell and La Roche, 

 3d edit., p. 77, Phil, 1833. 



4 Precis, &c., ed. cit., i. 77. 



* Dissert. Inaugural. Turin, 1823; cited in Edinb. Med. and Surg. Journal for July, 1834. 



