9 86 THE SENSES 



shown by Langley and Anderson that in the ins 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 con- 

 traction, 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 ' start- 

 ing 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. 



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

 local application of atropine causes temporary paralysis of accommoda- 

 tion 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 stimula- 

 tion 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 pupillae 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. i So, 713) which atropine paralyzes. The work of the mydriatics 

 can be undone by the miotics. Thus the dilatation produced by atro- 

 pine is removed by pilocarpine. 



Functions of the Iris. In vision the iris performs two chief 

 functions: (i) It regulates the quantity of light allowed to fall 

 upon the retina. The larger the aperture of a lens, the greater is 

 its collecting power, the more light does it gather in its focus. In 

 the eye, the area of the pupil determines the breadth of the pencil 

 of light that falls upon the lens. If this area was invariable, the 

 retina would either be ' dark from excess of light ' in bright sunshine, 

 or dark from defect of light in dull weather or at dusk. In order 

 that the iris may act as an efficient diaphragm it must be pig- 

 mented, and it is the pigment in it which gives the colour to the 

 normal eye. The vision of albinos, in whose eyes this pigment is 

 wanting, is often, though not invariably, deficient in sharpness. 

 There is always intolerance of bright light; and the same is true 

 in the condition known as irideremia, or congenital absence or 

 defect of the iris. 



