vi DIOPTKIC MECHANISM OF THE EYE 315 



of poisons, either introduced into the blood or injected into the 

 conjunctiva! sac, including atropine, duboisine, daturine, etc. 

 While the myotic poisons induce persistent accommodation of the 

 eye to near vision, the mydriatic poisons accommodate for distance ; 

 in other words, the former produce a spasm, the latter a relaxa- 

 tion of accommodation and of the pupillary movements. Cocaine 

 in small doses is a mydriatic, in large doses a myotic. 



To sum up, it may be said that the aperture of the pupil both 

 under normal and under abnormal conditions varies constantly, 

 owing either to oscillations in the tone of its muscles or to oscilla- 

 tions of the blood contained in its vessels. Sometimes this 

 normal dynamic state is so exaggerated that there is intermittent 

 contraction of the pupil, known to ophthalmologists as hippus, 

 which may be associated with an analogous state of the external 

 muscles of the eyeball, known as nystagmus. 



IX. The movements of the iris depend essentially on the 

 activity of its muscles. The sphincter of the pupil forms a ring 

 round the inner border of the iris which may vary in diameter, 

 according to the state of its contraction, between 0'6 and T2 mm. 

 The existence of the musculus dilatator pupillae was clearly 

 established by the anatomical researches of Grunert (1898) and 

 others, but it had previously been known from the physiological 

 experiments of Langley and Anderson (1892) on mammals. ' They 

 applied electrical excitation to a group of ciliary nerves, after 

 exposing a small portion of the sclera near the corneal edge, and 

 observed a local traction of the pupil outwards, due to the 

 simultaneous radial contraction of the corresponding portion of 

 the iris and sphincter. They also found that a bit of iris separated 

 from the rest by two radial incisions contracted on exciting the 

 corresponding nerves. In these experiments they ascertained by 

 means of the microscope that the pupillary dilatation was wholly 

 independent of the contraction of the blood-vessels of the iris. 



There is thus no doubt that there are two muscles of antagon- 

 istic action in the iris. According as the one or the other pre- 

 dominates, there is myosis or mydriasis. When both muscles 

 contract, the contraction of the sphincter predominates. 



According to the observations of Laqueur (1898) the sphincter 

 of the pupil is capable of very large excursions. The muscle fibre- 

 cells of which it is composed may shrink to ^th their length 

 during contraction. These observations recall the experiments 

 made by Griinhagen (1874) on the isolated sphincter of the iris of 

 the rabbit and the cat, when a strip of the muscle was connected 

 to a lever writing on a smoked drum. The experiment seems 

 important, and has not, so far as we are aware, been controverted 

 by other workers. The results may be summed up as follows : 



The sphincters both in rabbits and in cats react in the same way 

 to the influence of the external temperature. They shorten when 



