314 PHYSIOLOGY CHAP. 



occur with less or greater rapidity. To clear up the question 

 Ovio adopted a coefficient of pupillary dilatation, and found this 

 coefficient to be inversely proportional to the intensity of light. 



(b) The pupil, as we have seen, contracts reflex ly when the eye 

 is accommodated to vision of near objects, and when the eyes 

 converge. In such cases accommodation (contraction of ciliary 

 muscle) and convergence (contraction of recti interni) take place 

 more rapidly than the contraction of the sphincter of the iris. 

 The association of these three movements is not constant. 

 According to Ovio the pupil may react to accommodation alone, 

 that is, apart from convergence of the visual axes, or on con- 

 vergence alone. The reaction of the pupil in consequence of con- 

 vergence of the axes seems, however, greater than that which 

 occurs on accommodation. 



(c) The pupil is contracted during deep sleep, in the early 

 morning, in epileptic attacks, and in other nervous diseases. 



(d) Myosis further occurs in the early stages of chloroform 

 narcosis, and at almost every stage of poisoning with physostigmine, 

 nuiscariue, morphine, ami other drugs known as myotics. It is 

 difficult to be certain of the exact cause of the myosis in all 

 cases. Hyperaemia or increased flow of blood to the vessels of 

 the iris may induce it ; in fact, if the aqueous humour is drawn off 

 by paracenthesis of the cornea, myosis results (Hensen and 

 V< tickers). 



Reflex dilatation nf the pupil is produced by 



(a) Passage from light to darkness, or diminution of the 

 intensity of light. 



((3) Adaptation of the eye to distant vision, which depresses 

 the tone of the ciliary muscle, and makes the two visual axes 

 parallel. 



(y) Excitation of the nerves by any kind of stimulus that 

 produces pain (Cl. Bernard, Westphal). According to Schiff 

 and P. Foil, sudden, gentle, tactile impacts produce a transient 

 mydriasis. The dilatation of the pupil that accompanies dyspnoea 

 and great muscular exertion is probably due to the rhythmic and 

 continuous stimulation of the sensory nerves. Even under normal 

 conditions inspiration coincides with a slight pupillary dilatation, 

 and expiration with a slight contraction (Vigoureux). According 

 to certain observers (Hensen) minute oscillations in the diameter 

 of the pupil are visible at each arterial pulse. But these effects 

 are in obvious relation with the respiratory and arterial variations 

 in the blood-pressure and the rate of blood-flow in the vessels of 

 the iris, and have nothing to do with the pupil reflexes. 



(8) Psychical emotions, such as fear, surprise, and the like, are 

 also accompanied by dilatation of the pupil. 



(e) Lastly, mydriasis occurs in advanced stages of chloroform 

 narcosis and alcoholic intoxication, and as the effect of a number 



