318 PHYSIOLOGY CHAP. 



in the cat, which has a large ciliary ganglion, Marchi's method 

 shows no degeneration of the fibres of the ciliary nerve. 



The immediate centre for the constrictor fibres of the pupil is 

 the nucleus of the oculo-motor nerve. By exciting this in dogs, 

 Hensen and Volckers (1878) found that it lay behind the centre 

 for the ciliary muscle, which contracts on accommodation. 



The centre for the dilatation of the pupil lies in the lower part 

 of the cervical cord (Steil, 1894). 



Both centres are normally in tonic excitation, so that after 

 section of the oculo-motor the pupil dilates, after section of the 

 sympathetic it contracts. The tone of the constrictor fibres is 

 mainly reflex, as it ceases after division of the optic nerve (Knoll) ; 

 yet the pupillary myosis of sleep demonstrates the existence of 

 an automatic tone as well. The tone of the dilatator fibres is 

 predominantly automatic. 



The tonic action of these centres is normally equally active on 

 both sides. The reflexes aroused by light or darkness also take 

 place simultaneously and equally in both eyes, even when the 

 positive or negative stimulus affects one eye only. In this case 

 the reflex that occurs in the non-stimulated eye is called indirect 

 or consensual. 



. Garten (1897), in order to study the course of its reflex, 

 photographed the pupil on sensitised moving paper, and succeeded 

 in demonstrating that as the effect of darkness it dilated rapidly 

 in the first 5 sees., then more slowly, and finally reached its 

 maximal dilatation, at which it remained for several hours. 



If, after being kept in the dark for a minute, the eye is 

 suddenly illuminated by a magnesium flame, the pupil contracts 

 after a latent period of about 0'5 sees., reaches the maximum of 

 contraction in about 4 sees., remains stationary for 6 sees., and 

 finally relaxes at first rapidly and then more slowly. The con- 

 traction of the pupil is greater and more prolonged in proportion 

 with the adaptation of the eye to darkness, i.e. with its sensibility 

 to light. 



Schirmer noted that in passing from darkness into an illumin- 

 ated room the pupil at first contracts rapidly, then dilates slowly, 

 and only resumes its normal size after two to four minutes. Garten 

 further saw that a slow increase of luminous intensity, within 

 certain limits, produces no change in the pupil, while a rapid 

 increase of light to the same intensity causes marked pupillary 

 constriction. 



Bellarminoff, by the same photographic method as Garten, 

 studied the dilatation of the pupil that ensues in animals on stimu- 

 lating the sympathetic after dividing it (direct dilatation), and 

 that consequent on stimulation of the central trunk of the sciatic 

 or other sensory nerve (reflex dilatation). The two reactions are 

 different in type, as appears from Figs. 144, 145. In the first 



