THE CORNEA, IRIS AND AQUEOUS HUMOR 813 



In the case of the light reflex, the stimuli are received upon the 

 retina, whence they are conveyed over the peripheral optic tract to 

 the secondary or reflex optic center, situated in the anterior corpora 

 quadrigemina next to the aqueduct of Sylvius. 1 To effect pupillar 

 constriction they are transferred from here to the oculomotor nerve 

 and the ciliary ganglion and nerves. Pupillar dilatation, on the other 

 hand, is accomplished with the aid of the autonomic fibers and hence, 

 these impulses must be diverted from the secondary optic center into 

 the sympathetic system proper. Some authors also hold that the 

 retina gives rise to two kinds of fibers, one group of which has to do 

 with visual sensations proper and the other solely with the differences 

 in the intensity of the light. 2 The time which is required for this 

 reflex response of the iris, has been estimated at 0.04 to 0.05 second. 



In man, as well as in those animals in which the optic fibers decus- 

 sate in part, the light reflex is bilateral, so that light falling into one 

 eye also gives rise to a diminution in the size of the pupil of the oppo- 

 site organ. This is not ^the case in such animals as the horse, owl and 

 rabbit, in which the crossing is complete. 3 Furthermore, it has been 

 noted that the substance of the iris, and especially in the lower forms, 

 is extremely sensitive to light. Even small pieces of the iris of the frog 

 or eel may be made to contract by simply permitting a beam of light to 

 fall upon them. 4 Clinically, the power of reaction of the pupils is 

 usually tested by shading one eye in such a manner that its pupillar 

 orifice can be observed beneath the cover. If the shaded eye is then 

 uncovered, its pupil will be seen to constrict. The other eye also 

 responds but not so intensely. This implies that the direct reaction 

 to light is usually more profound than the consensual, as practised in 

 this test. 



From this discussion it may be gathered that the light reflex is 

 abolished whenever the aforesaid reflex arc is broken at any point of 

 its course. This calls to our minds the important fact that it is absent 

 in tabes dorsalis (locomotor ataxia) and general paresis, while the 

 accommodation reflex is preserved. This phenomenon is known as the 

 Argyll-Robertson sign. Its explanation is not difficult if it is remem- 

 bered that the nervous paths required for these two reflexes are totally 

 different. Thus, the afferent arc in the case of the light reflex is 

 formed by the optic nerve, whereas that concerned with the accomoda- 

 tion reflex is formed by the afferent fibers from the muscles of the eye. 

 Inasmuch as the disease of tabes dorsalis is characterized by a pro- 

 gressive degeneration of the different spinal roots and tracts, it cannot 

 surprise us to find that similar changes are finally induced in the optic 

 path, thereby gradually blocking the impulses from the retina. At 



1 Hass, Archiv fur Augenheilkunde, Ix, 1908, 327. 



2 Behr, Archiv fur Ophthalmologie, Ixxxvi, 1913, 468. 



3 Steinach, Pfluger's Archiv, xlvii, 1890, 313. 



4 Arnold, Physiologie, ii, 1847; also see: Steinach, Pfluger's Archiv, lii, 1892, 

 495. 



