ACTIONS OF THE IRIS. 99 



thalami, occasions divergence and dilatation ; on excita- 

 tion of the outer part of either anterior tubercle, both 

 eyeballs are rotated to the opposite side. 



The iris receives, in addition to sensory and vascular 

 nerves, constrictor nerves, which are distributed to the 

 sphincter pupillse, and dilatator fibres, which are believed 

 to terminate in muscular structures of corresponding 

 function. Every excitation of the retina by light is fol- 

 lowed, after an interval of about half a second, by con- 

 traction of the pupils. Both pupils respond to excitation 

 of one retina. In the frog, the iris continues to respond 

 to excitation of the retina, even after the eye has been 

 removed from the body. In accommodation for near 

 vision, the contraction of the tensor choroideae is normally 

 associated with convergence of the visual axes and nar- 

 rowing of the pupil. The dilatator nerves of the iris are 

 derived immediately from the trigeminus, ultimately from 

 the sympathetic system, for excitation of the upper cer- 

 vical ganglion produces dilatation, and destruction of it 

 narrowing of the pupil. Dilatation is also produced re- 

 flexly by excitation of any sensory nerve : in all cases it 

 is associated with vascular contraction (see Vascular Ner- 

 vous System), proptosis, widely-opened eyelids, and re- 

 traction of the membrana nictitans. These effects are 

 weakened, but not annulled, by extirpation of the upper 

 ganglion of the sympathetic. Corresponding phenomena 

 are observed in dyspnoea and during violent muscular 

 efforts. In deep sleep, the visual axes converge, and the 

 pupil is contracted. Certain alkaloids (called mydriatics) 

 produce lasting dilatation of the pupil, associated with 

 complete relaxation of the tensor of the choroid. Others 

 (called myotics) have the opposite effect ; in the former 

 case, the action is known to have its seat in the eyeball 

 itself. Convergence of the visual axes, and narrowing of 

 the pupil are so associated, that one of them cannot be 

 normally accomplished without the other. Accordingly, 



H 2 



