792 A MANUAL OF PHYSIOLOGY 



iris. In all probability they act directly upon dilator 

 muscular fibres. It has, indeed, long been known that in 

 the iris of the otter and of birds a radial dilator muscle 

 exists; and it has been shown by Langley and Anderson 

 that in the iris of the rabbit, cat, and dog, the presence of 

 radially arranged contractile substance, different it may be 

 in some respects from ordinary smooth muscle, must be 

 assumed. Reflex dilatation of the pupil through the sympa- 

 thetic fibres is caused in man by painful stimulation of the 

 skin, by dyspnoea, by muscular exertion, and in some indi- 

 viduals even by tickling of the palms. In animals the stimu- 

 lation of naked sensory nerves has the same effect. The 

 ' starting of the eyeballs from their sockets/ which the 

 records of torture so often note, is due to a similar reflex 

 excitation of the sympathetic fibres supplying the smooth 

 muscle of the orbits and eyelids. 



The statement has been made that in addition to the sympathetic 

 dilators of the pupil, dilating fibres pass out directly from the bulb 

 along the fifth nerve ; and after section of the cervical sympathetic 

 or excision of the superior cervical ganglion, reflex dilatation can still 

 be caused. Stimulation of certain areas on the cortex of the frontal 

 lobe of the cerebrum (p. 742) causes slight dilatation even after the 

 sympathetic has been divided. But it is not known whether this is 

 due to inhibition of the pupillo constrictor fibres in the third nerve 

 or to excitation of cerebral pupillo-dilator fibres. The reflex centre 

 for dilatation of the pupil is in the medulla oblongata. The lower 

 cervical and upper thoracic portion of the spinal cord has received 

 the name of the cilio spinal region from its relation to the pupillo- 

 dilator fibres. It must not be looked upon as a centre in any proper 

 sense of the term, but rather as the pathway by which these fibres 

 pass down from the bulb, and where they may accordingly be tapped 

 by stimulation. 



That, in addition to the cerebral centre for the constrictor 

 and the bulbar centre for the dilator fibres, there exists 

 within the eye some local mechanism which controls the 

 muscles of the iris and regulates the size of the pupil is 

 rendered certain by many facts. The excised eye of a frog 

 or an eel constricts its pupil on exposure to light, and dilates 

 it in the dark. It is said that even the isolated iris of the 

 eel contracts to light ; and it is known, although here the 

 explanation is less difficult, that the iris both of cold- and 

 warm-blooded animals contracts in warm, and dilates in 

 cold normal saline solution. The local application of atropia 



