SENSE OF SIGHT. 863 



since they cannot be produced through the exercise of the will. When 

 a ray of light falls upon the retina it leads to contraction of the pupil, 

 in which the optic nerve is the afferent nerve, the oculo-motor nerve the 

 efferent nerve, while the centre lies in some place in the brain below the 

 corpora quadrigemina. This is proven by the fact that when the optic 

 nerve is divided intense light no longer produces contraction of the pupil, 

 while, also, the division of the third pair renders the pupil insensitive 

 to light, and stimulation of the peripheral portion of the .oculo-motor 

 leads to contraction of the pupil. So, also, stimulation of the floor of 

 the aqueduct of Sylvius will, likewise, if the oculo-motor nerve be intact, 

 lead to contraction of the pupil. 



In contradistinction to what will be found in many instances, two 

 symmetrically disposed centres are not to be found in the brain for gov- 

 erning the movements of the pupil, since in normal conditions a stimulus 

 applied to one optic nerve will lead to contraction of both pupils, while, 

 also, stimulation of the centre will react on both eyes. 



In addition to the oculo-motor nerve, the iris receives nerve-fibres 

 from the short ciliary nerves coming from the ophthalmic ganglion, 

 which is connected by its root with the third nerve, the cervical sympa- 

 thetic nerve, and the nasal branch of the ophthalmic division of the fifth 

 nerve. It has been mentioned that section of the cervical sympathetic 

 causes contraction of the pupil, and stimulation of the cervical sympa- 

 thetic dilatation of the pupil. It is evident that these effects on the 

 pupil are directly opposite to those seen in the blood-vessels on stimu- 

 lation of the sympathetic nerve. 



The centre for the dilatation of the pupil likewise lies in the front 

 part of the floor of the aqueduct of Sylvius, while a second or inferior 

 centre, the so-called cilio-spinal centre, lies in the lower cervical portion 

 of the cord and extends downward to the first or third dorsal vertebra. 

 This centre may be reflexly stimulated bj 7 irritation of various sensory 

 nerves, when, of course, dilatation of the pupil will take place. It is 

 likewise stimulated by the blood in dyspnoea, and also a single centre 

 governs the movements of both pupils, for if one retina be shaded both 

 pupils will dilate. The cilio-spinal centre is in connection with the upper 

 centre for the dilator of the pupil through fibres passing through the 

 lateral columns of the cord, and it, together with the inferior centre, 

 reacts to the same stimuli. 



When the sympathetic nerve is divided in the neck the pupil con- 

 tracts. This contraction is accompanied by a great increase in the vascu- 

 larity of the iris from the consequent paralysis of the walls of its 

 blood-vessels. This at one time was supposed to be sufficient to explain 

 the production of contraction of the pupil after section of the sym- 

 pathetic. Various other conditions which lead to an increased blood 



