634 



COMPARATIVE PHYSIOLOGY. 



pillary aperture, its relaxation to an enlargement, provided the 

 rest of the iris were highly elastic. 



The conclusions in regard to the innervation of the iris rest 

 largely upon the results of certain experiments which we shall 



Brain above medulla 



Optic centre— 



Betina 



Iris 



Sympathetic nerve to 

 radiating fibres 



Spirted dilator centre— 



Pig. 383.— Diagram to illustrate innervation of the iris. Dotted lines indicate general 

 functional connection (correlation). Course of impulses indicated by arrows. 



now briefly detail : 1. When the third nerve is divided, stimu- 

 lation of the optic nerve (or retina) does not cause contraction 

 of the pupil as usual. 2. When the optic nerve is divided, light 

 no longer causes a contraction of the pupil, though stimulation 

 of the third nerve or its center in the anterior portion of the 

 floor of the aqueduct of Sylvius does bring about this result. 

 3. Section of the cervical sympathetic is followed by contrac- 



