842 



THE IRIS. 



The size of the pupil increases from the first month of life up to from the 

 third to the sixth year; and with it also the amplitude of reaction decreases, 

 though more slowly. 



With regard to the size of both pupils it may be remarked that when 

 there is semidecussation of the optic nerves, the pupils are always of the 

 same size, and they react symmetrically (man, cat). In animals in 

 which the decussation is total (horse, owl), and in those that have only a 

 few uncrossed fibers in the optic tract (rabbit), the pupillary reflex is 

 confined to the eye tested. 



The iris has two muscles: the sphincter, which surrounds the pupil 

 and is supplied by the oculomotor nerve; and the dilator of the 

 pupil, supplied chiefly by the cervical sympathetic and the trigeminus. 

 The two muscles are antagonistic; the pupil dilates, therefore, after 

 paralysis of the oculomotor, by the predominance of the sympathetic; 

 conversely, it contracts after excision of the sympathetic. Simultane- 

 ous irritation of both nerves causes the pupil to contract; the excita- 

 bility of the oculomotor is consequently the greater. 



According to Arnstein and A. Mayer all the nerve-fibers lose their myelin- 

 sheaths after a short course. Most of the motor fibers near the sphincter consist 

 of naked bundles of fibers. Under the anterior epithelium there is a network 

 of exceedingly fine sensory nerves. Numerous fibers pass to the capillaries 

 and arteries as vasomotor nerves. 



The movements of the iris take place under the following conditions : 



1. Irritation of the retina by light causes a contraction of the pupil corre- 

 sponding to the intensity and extent of the irritation. Irritation of the optic 

 nerve itself has the same effect. This movement is a reflex action transferred 

 to the path of the oculomotor nerve The center is situated in the anterior 

 pair of quadrigeminate bodies near the aqueduct of Sylvius. After section 

 of the optic nerve the pupil dilates and subsequent section of the oculomotor 

 causes no further dilatation. In the dark the pupil dilates, at first rapidly, 

 later more slowly. Immediately after the darkening an illumination must 

 have considerable strength to cause pupillary contraction. After the eye has 

 become accustomed to the darkness, a weaker light is sufficient. A flash of 

 lightning following a long period of darkness produces strong and prolonged 

 contraction. A slow increase in illumination is almost without effect. 



2. The center for the dilator fibers of the pupil is irritated by a state of the 

 blood causing dyspnea. If the dyspnea passes into asphyxia, the dilatation of 

 the pupil diminishes. Previous section of the peripheral dilator fibers makes 

 these reactions impossible. Sudden anemia also has a stimulating action. 



_ 3. The center, as well as the ciliospinal region of the cord subordinated to 

 it, is also susceptible to reflex irritation. Painful excitation of the sensory nerves 

 produces dilatation of the pupils and protrusion of the eyeballs, as was demon- 

 strated by the ancient acts of torture. Labor-pains, loud noises in the ear, irrita- 

 tions of the nerves of the sexual organs, and even slight tactile, sensations have 

 the same effect. According to Bechterew, these results are due to an inhibition 

 of the light-reflex, in the sense expressed on p. 731. 



4. The condition of the blood-vessels of the iris has an important influence 

 on the size of the pupil. Everything that increases their injection contracts 

 the pupil, while everything that diminishes the amount of blood dilates the pupil. 

 The pupil is contracted, therefore, by forced expiration, which prevents the 

 return of blood from the head; momentarily by each pulsation of the heart (by 

 diastolic filling of the arteries); by decrease of intraocular pressure, for example 

 after puncture of the anterior chamber, because more blood can enter the vessels 

 of the iris, owing to the diminished intraocular pressure; further, by paralysis 

 of the vasomotor fibers of the iris. Conversely, the pupil is dilated by conditions 

 the reverse of those already mentioned, and also by strong muscular exertion, 

 during which blood rushes into the dilated muscular branches, and further, when 

 death takes place. The influence of the amount of blood accounts also for the 

 fact that the pupil when dilated by atropin becomes narrower as soon as the 

 superior cervical sympathetic ganglion, which supplies a part of the vasomotors 



