THE NERVOUS SYSTEM 517 



Mixed Group. 



This group, it will be remembered, resembles spinal nerves in 

 consisting of a sensory and motor portion, with a ganglion on the 

 sensory root. 



Third Pair, or Motor Oculi. — This nerve has a deep-seated 

 origin in the anterior corpora quadrigemina and peduncles of 

 the cerebrum. It supplies with motor power all the muscles of 

 the eyeball, including the retractor and muscle of the upper lid, 

 but does not supply the external rectus nor the superior oblique. 

 Through its connection with the ciliary ganglion it supplies 

 fibres to the sphincter of the iris and the ciliary muscle. At 

 its origin this nerve is connected with the two other motor nerves 

 of the eyeball — viz., the fourth and sixth pairs. Division of 

 the third pair causes the eye to turn downwards and outwards, 

 owing to the unbalanced action of the superior oblique and 

 external rectus muscles. There is also depression of the upper 

 lid, immobility of the eyeball, and dilatation of the pupil. The 

 sense of direction, and the reflex tonus of the eyeball muscles, 

 are also given by these nerves. The movement of the eyeball 

 muscles will be considered in detail later. 



Fourth Pair, or Pathetic. — This is the smallest cranial nerve. 

 It arises immediately behind the corpora quadrigemina in the 

 anterior cerebellar peduncle. The deep-seated origin is from a 

 nucleus in the floor of the cerebral aqueduct. Sisson* points out 

 that the fibres of this nerve decussate totally in the anterior 

 medullary velum, so that the fibres for the left eye are derived 

 from the right brain. The fourth cranial nerve supplies only one 

 muscle — viz., the superior oblique of the eyeball. 



Sixth Pair, or Abducens. — This nerve arises from the bulb 

 behind the pons, and external to the pyramid. It supplies the 

 external rectus or abductor muscle of the eyeball, the muscle 

 of the third eyelid (membrana nictitans), also the deep 

 recti. 



Fifth Pair, or Pars Trigemini. — The large sensory root of this 

 nerve arises from the pons, close to the middle peduncle of the 

 cerebellum, and at the foramen lacerum has upon it a large 

 ganglion known as the Gasserian. The deep-seated origin of 

 the sensory root is from the trigeminal nucleus, which, according 

 to Dexler, f extends from the pons to the sixth cervical segment 

 of the cord, and is known as the spinal tract of the trigeminus. 

 The connections of the trigeminal sensory root in the brain are 

 with the thalamus and cerebral cortex of the opposite side. In 

 ungulates it is also, according to Wallenberg,! connected with the 



* ' Veterinary Anatomy.' f Sisson, op. cit. 



% Sisson, op. cit. 



