THE SENSES 911 



Stimulation of the cervical sympathetic causes marked dilata- 

 tion of the pupil, even when the third nerve is excited at the 

 same time. The pupillo-dilator fibres do not act by constricting 

 the bloodvessels of the iris. For dilatation of the pupil can be 

 caused in a bloodless animal by stimulating the sympathetic. 

 And even when the circulation is going on, a short stimulation 

 of the sympathetic causes dilatation of the pupil without vaso- 

 constriction, while with longer excitation the dilatation of the 

 pupil begins before the narrowing of the bloodvessels. Nor 

 does it seem possible to accept the view that the sympathetic 

 fibres are inhibitory for the sphincter muscle of the iris. 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. 

 Both the constrictor and the dilator muscles of the iris are 

 normally in a condition of greater or less tonic contraction, so 

 that the size of the pupil at any given moment depends on the 

 play of two nicely balanced forces. Reflex dilatation of the 

 pupil through the sympathetic fibres is caused in man by painful 

 stimulation of the skin, by dyspnoea, by muscular exertion, and 

 in some individuals even by tickling of the palms. In animals 

 the stimulation 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 excita- 

 tion of the sympathetic fibres supplying the smooth muscle of 

 the orbits and eyelids. 



Action of Drugs on the Function of the Intrinsic Eye Muscles. 

 The local application of atropine causes temporary paralysis of 

 accommodation and dilatation of the pupil. When the third nerve 

 is divided, the pupil dilates ; it dilates still more when atropine is 

 administered after the operation. Dropped into one eye in small 

 quantity, atropine only produces a local effect ; the pupil of the other 

 eye remains of normal size, or somewhat constricted on account of 

 the greater reflex stimulation of its third nerve by the greater 

 quantity of light now entering the widely-dilated pupil of the 

 atropinized eye. Even in the excised eye the effect of the drug is 

 the same. Introduced into the blood atropine causes both pupils 

 to dilate. Its action is to paralyze the endings of the oculo-motor 

 fibres to the sphincter pupillse and ciliary muscle. Other mydriatic, 

 or pupil-dilating drugs, are cocaine, daturine, and hyoscyamine. 

 Physostigmine or eserine, pilocarpine, and muscarine are the chief 

 miotics, or pupil-constricting substances. They also cause spasm of 

 the ciliary muscle, and inability to accommodate for distant objects. 

 They act by stimulating the structures (nerve-endings) (see pp. 166, 

 635) which atropine paralyzes. The work of the mydriatics can be 

 undone by the miotics. Thus the dilatation produced by atropine is re- 

 moved by pilocarpine. Adrenalin, when injected intravenously, causes 



