CRANIAL NERVES. 435 



without "becoming connected with the brain. This nerve 

 presides exclusively over the sense of vision, and by these 

 crossings of the fibres, and the resulting connection of the 

 centres implicated, the unity of vision is most satisfactorily 

 accounted for. These nerves likewise give rise to the reflex 

 action by which the pupils contract. They are utterly 

 insensible to pain, and if irritated, give rise only to the 

 impression of sparks or vivid flashes of light. If divided, 

 vision is lost, and the iris ceases to contract through loss of 

 its accustomed stimulus of light. 



AUDITORY NERVES. The nerve of hearing originates 

 from a grey nucleus on the floor of the fourth ventricle, 

 close to the median line. In passing outwards, its fibres 

 are a good deal scattered, and in the intervals pass nume- 

 rous longitudinal bundles, which, with the intimate con- 

 nection between its nucleus and those adjacent, in the 

 opinion of Van der Kolk, enables this nerve to act in a 

 reflex manner upon these, and on the occurrence of a loud 

 noise, to place the whole body as it were instinctively in a 

 position of defence. The two nuclei are, moreover, closely 

 united by transverse fibres, so that a sound falling on the 

 two ears simultaneously produces a single, and not, as 

 might have been expected, a double impression. The 

 ramifications of these nerves in the internal ear present 

 numerous ganglionic cells, so that this, like the retina, is 

 truly a nervous centre. These nerves can convey sound 

 only, and may be destroyed without producing pain. In- 

 flammation existing in the internal ear or adjacent parts, 

 gives rise to such perceptions as buzzing, ringing, rushing 

 of water, &c. It is remarkable that mechanical destruction 

 of the internal ear on one side leads to the phenomenon of 

 the animal turning continuously to that side. 



Motor Oculi. The third pair, the motor nerve of the eye, 



