MOVEMENTS OF THE IRIS, 759 



3. To a certain extent it supports the action of the ciliary muscle. 



Muscles and Nerves. The iris is usually described as being provided with two 

 sets of muscular fibres the sphincter, which immediately surrounds the pupil and 

 is supplied by the oculomotorius ( 345, 2), and the dilator pupillae (p. 736), 

 supplied chiefly by the cervical sympathetic ( 356, A, 1), and the trigeminus 

 ( 347, 3). Both muscles stand in an antagonistic relation to each other ( 345), 

 the pupil dilates moderately after section or paralysis of the oculomotorius, owing 

 to the contraction of the dilator fibres which are supplied by the cervical sym- 

 pathetic ; conversely, the pupil contracts when the sympathetic is divided or 

 extirpated (Petit, 1727). When both nerves are stimulated simultaneously, the 

 pupil contracts, so that the excitability of the oculomotorius overcomes the sym- 

 pathetic. 



[The existence of a dilator pupillse muscle is not universally recognised, and in fact some 

 observers doubt its existence. The muscular nature of the radial fibres in the posterior limiting 

 membrane of the iris is denied by Griinhagen, while Koganei regards these as in no case 

 muscular, and the dilating fibres as represented by fibres radiating from the iris. These fibres 

 are well-developed in birds and the otter, exist in traces in the rabbit, and are absent in man. 

 Gaskell points out that in this case the size of the pupil must in part depend on the elasticity 

 of the radial fibres of the iris, while the dilator nerve-fibres must act on the sphincter fibres, 

 causing them to relax. Gaskell groups the sphincter of the iris with those muscles "supplied 

 by two nerves of opposite character, the one motor, the other inhibitory." The dilatation of 

 the pupil caused by stimulation of the cervical sympathetic is usually explained by the 

 hypothesis that this nerve contains motor fibres, which act on the dilator fibres. Griinhagen 

 thought that it might be due chiefly to the constriction of the blood-vessels of the iris ; 

 Gaskell suggests that the nerve acts on the sphincter muscle, and is the inhibitory nerve of 

 that muscle, dilatation taking place because the sphincter is normally in a condition of tonic 

 contraction, and also because the posterior limiting membrane is elastic] 



Nerves. Arnstein and A. Meyer have studied the mode of termination of the nerve-fibres in 

 the iris. All the medullated nerve-fibres lose their white sheaths after a time ; most of the 

 fibres {motor) in the region of the sphincter consist of naked bundles of fibrils. A network of 

 very delicate sensory nerves lies under the anterior epithelium. Numerous fibrils pass to the 

 capillaries and arteries as vaso-motor nerves. [Many ganglionic cells are intercalated in the 

 course of the fibres. ] 



Movements of the iris occur under the following conditions : 



1. Action of light on the retina causes, (according to its intensity and amount), a correspond- 

 ing contraction of the pupil ; the same effect is produced by stimulation of the optic nerve 

 itself {Herbert Mayo, f 1852). This movement is a reflex act, [the afferent nerve being the 

 optic and the efferent the oculomotorius ; the impulse is transferred from the former to the 

 latter in a centre situated somewhere below the corpora quadrigemina (fig. 544, C)]. The 

 older observers locate the centre in the corpora quadrigemina, the recent observers in the 

 medulla oblongata (p. 660). Both pupils always react, although only one retina be stimulated ; 

 generally under normal circumstances both contract to the same extent (Bonders), owing 

 the intercentral communication [coupling] of the two pupillo-constricting centres. [This 

 called consensual contraction of the pupil.] After section of the optic nerve the pupil dilates 

 and subsequent section of the oculomotorius no longer produces any further dilatation 

 (Knoll). 



2. The centre for the dilator fibres of the pupil (pupillo-dilating centre) is excited by 

 dyspnceic blood ( 367, 8). If the dyspnoea ultimately passes into asphyxia, the dilatation of 

 the pupil diminishes. Of course, if the peripheral dilating fibres ( 247, 3) [e.g., the 

 sympathetic nerve in the neck] be previously divided, this effect cannot take place, as the 

 dyspnceic blood acts on the centre and not on the nerve-fibres. 



3. The centre, as well as the subordinate " cilio-spinal region" of the spinal cord ( 362, 1), 

 is also capable of being excited reflexly ; painful stimulation of sensory nerves, in addition to 

 causing protrusion of the eyeballs ( 347), a fact proved in the case of persons subjected to 

 torture, produces dilatation of the pupils (Arndt, Bernard, Westphal, Luchsinger) ; while a 

 similar effect is caused by labour pains, a loud call in the ear, stimulation of the nerves of the 

 sexual organs, and even by slight tactile impressions (Foa and Schiff). According to Bechterew, 

 the foregoing results are due to inhibition of the light-reflex in the sense expressed in 361, 3. 



4. The condition of the blood-vessels of the iris influences the size of the pupil ; all condi- 

 tions causing injection or congestion of these vessels contract the pupil, all conditions diminish- 

 ing them dilate it. The pupil, therefore, is contracted by forced expiration, which prevents the 

 return of venous blood from the head, momentarily by every pulse-beat, owing to the diastolic 

 filling of the arteries ; diminution of the intraocular pressure, e.g., after puncture of the anterior 



to 



!S. 



