1044 VISION. 



tracts to light in an extirpated eyeball. It has been disputed whether 

 this is due to direct action on the iris or to an intra-ocular reflex. 

 Steinach l has shown that the reaction is greatly influenced by the state 

 of the eye, and is very marked when the animal has previously been 

 kept for some time in the dark. The iris of the extirpated eye will then 

 contract to light, when the whole of the posterior pole of the eye, includ- 

 ing the retina, has been removed by cauterisation. With local stimula- 

 tion of the pupil by light, Steinach found that the contraction began at 

 the part stimulated and slowly spread to other parts. The reaction, 

 however, did not occur on stimulation of the peripheral part of the iris, 

 and was limited to the part near the pupillary border. In the eel, sala- 

 mander, and frog, Steinach found that the muscle fibres of the sphincter 

 were pigmented, fine yellowish brown pigment granules appearing to lie 

 between the muscle fibrillee, and he ascribes the influence of light to its 

 direct action on the pigment. Steinach investigated the influence of the 

 different parts of the spectrum, and found that this corresponded with the 

 absorption of the pigment, the maximum effect being in the green about 

 F, while the red end of the spectrum as far as C produced no contraction. 

 Mechanism of dilatation. There has been till recently much un- 

 certainty as to the cause of the dilatation of the pupil. It has been ascribed 

 to the contraction of radial muscular fibres, to inhibition of the sphincter, 

 and to vaso-constriction, acting either by emptying the iris of blood, or by 

 producing longitudinal contraction of its radial arteries. Langley and 

 Anderson 2 have shown that there is a substance in the iris which con- 

 tracts radially, but have left it an open question whether this substance 

 is in the form of unstriped muscle fibres. They found that local stimu- 

 lation of the sclerotic produced dislocation of the pupil, which could only 

 be explained by radial contraction, and that local dilatation of the pupil 

 might be observed to occur simultaneously with local contraction of the 

 sphincter. They found that a strip of iris, isolated except at its ciliary 

 attachment, shortened on stimulation of the cervical sympathetic to as 

 much as half its length ; and they showed that this shortening was not 

 accompanied by any appreciable change in the blood vessels. Heese, 3 

 about the same time, took tracings of the contraction of radial strips of the 

 iris with the sphincter removed, and on stimulation of the cervical sympa- 

 thetic obtained curves, corresponding, as regards form and latent period, 

 with those of unstriped muscle fibres. The question still remains, whether 

 this radial contraction is accompanied by simultaneous inhibition of the 

 sphincter. Langley and Anderson failed to obtain any evidence of relaxa- 

 tion of the sphincter on stimulation of the sympathetic. Waymouth Keid 4 

 has, however, found definite differences in the electrical variation of a de- 

 marcation current, when the electrodes are placed concentrically near the 

 pupil and radially near the periphery of the iris. When the third nerve 

 was stimulated, there was a negative variation in the concentric position 

 and a positive variation in the radial position ; when the sympathetic 

 was stimulated, the concentric position gave a positive, the radial gave a 

 negative, variation. Keid concludes that contraction of the sphincter is 

 accompanied by inhibition of the radial contractile substance, and that 

 contraction of the latter is accompanied by inhibition of the sphincter. 



1 Arch./, d. ges. Physiol., Bonn, 1892, Bd. lii. S. 495. 



2 Journ. Physiol. , Cambridge and London, 1892, vol. xiii. p. 564. Refer to this paper 

 for a full account of the literature of the subject. 



3 Arch.f. d. gcs. Physiol., Bonn, 1892, Bd. lii. S. 535. 



4 Journ. Physiol., Cambridge and London, 1894-5, vol. xvii. p. 433. 



