The Nervous System. 285 



the eyeball ; it supplies with motor power all the muscles 

 (excepting the external rectus and the superior oblique), 

 also the muscle of the upper lid. Through its connection 

 with the lenticular ganglion it supplies fibres to the iris 

 and ciliary muscle ; it is also connected at its origin with 

 two other motor nerves of the eyeball, viz., the fourth and 

 sixth pairs. 



The deep-seated origin of the third pair is from the corpora 

 quadrigeniina and peduncles of the cerebrum. Division of 

 the nerve causes the eye to turn the temporal side of the 

 pupil upwards and outwards, owing to the unbalanced 

 action of the superior oblique and external rectus ; there is 

 also depression of the upper lid, immobility of the eyeball, 

 and dilatation of the pupil. The action of the third pair 

 will be discussed again in connection with the physiology 

 of sight. 



Fourth Pair, or Pathetic. — The motor nerve of the 

 superior oblique muscle of the eyeball ; it has a deep-seated 

 origin in the valve of Vieussens. 



Fifth Pair, or Pars Trigemini, resembles a spinal nerve in 

 having two roots, a motor and sensory ; and the resemblance 

 is carried still further by the sensory root having a large 

 ganglion on it, the so-called Gasserian. The motor root 

 arises from the trigeminal nucleus, and is connected with 

 the cerebral cortex on the opposite side. The sensory 

 fibres arise from the sensory trigeminal nucleus ; but 

 fibres in connection with the origin of this branch can be 

 traced upwards into the cerebrum and cerebellum, and 

 downwards into the grey matter of the cord ; further, 

 it has also connections with all the nerves arising from 

 the medulla, excepting the abducens. In this way can be 

 explained the extensive connections and varied reflex acts 

 of the fifth pair. 



The motor branches, or inferior maxillary division of the 

 fifth, supply the whole of the muscles concerned in masti- 

 cation ; by some it is considered that sensory fibres also 

 exist in this branch. 



The sensory branch, or superior maxillary division, 



