510 



PHYSIOLOGY 



INNERVATION OF THE IRIS. Before the work of Langley and Anderson 

 on the iris there was doubt as to the method by which dilatation was brought about : 

 some thought that it was due to inhibition of the sphincter, thus allowing the iris to 

 open because 'of the radial elastic fibres which it contained ; others that it was due 

 to the emptying of the iris of blood from the contraction of the arterioles following 

 stimulation of the sympathetic ; others again that the cause was the longitudinal 

 contraction of the radial arteries. But Langley and Anderson showed that a radial 

 strip of iris, isolated except at its ciliary attachment, shortened to half its length 

 \\hen the cervical sympathetic was stimulated. It lias been found further that 

 local stimulation of the iris near its periphery causes a local dilatation of the 

 pupil, and that cutting the sympathetic causes lasting constriction of the pupil. 



FIG. 255. Effect on iris of cat of local stimulation. 



The first effect, as in A, is to cause contraction of the constrictor pii|>ill;i> helow 

 the electrodes, and this is succeeded in B by a strong localised contraction of the 

 radiating fibres. (LANGLEY and ANDERSON.) 



During these experiments they proved by microscopic examination that the dilata- 

 tion of the pupil was wholly independent of the contraction of the blood v ss< -Is "I 

 the iris, and that draining the animal of blood did not influence the contraction of the 

 iris. Later it was proved by histological technique that there are radial inuselr 

 fibres in the iris. These are poorly developed in mammals but are well marked in hinls 

 and in the otter. These facts together prove definitely that there exists a dil-itoi 

 muscular mechanism in the iris. 



The sphincter mnsele is supplied by nerve fibres which arise from the 

 upper portion of the 3rd nerve nucleus in the ventral part of the Sylvian 

 grey matter (see Figure 247). They travel down in tin- nerve as far as the 

 ciliary (or lenticular) ganglion which is situated behind the eye close .to the 

 optic nerve. Here a branch from t lie nerve enters the ganglion to anasto- 

 mose with its nerve cells. These nerve cells send off numerous small nerves 

 called the short ciliary nerves (Fig. 253) which enter minute apertures in the 

 sclera arranged in a ring round the optic nerve. Having entered the peri- 

 choroidal lymph space the nerves form a plexus from which are supplied the 

 local blood vessels, the ciliary muscle (thus causing accommodation) and 



