THE TRIGEMINAL NERVE 61 



the lateral rectus. When at rest, the paralysed eye looks 

 downwards and laterally, and the patient, therefore, tends to 

 walk with the head rotated to the opposite side so as to enable 

 him to look directly forwards. In addition to the disability pro- 

 duced by the paralysis of the ocular muscles, ptosis results 

 owing to paralysis of the levator palpebrae superioris. In an 

 endeavour to counteract this condition, the patient extends the 

 head on the trunk and actively contracts the frontalis muscle. 



Owing to the paralysis of the sphincter pupillae, the pupil is 

 widely dilated by the unopposed dilator muscle (p. 67). The 

 ciliary muscle is also affected and, therefore, the accommoda- 

 tion reflex is lost in addition to the light reflex. 



The Fifth or Trigeminal Nerve contains both motor and 

 sensory fibres, and it therefore possesses two nuclei. The 

 MOTOR NUCLEUS is an elongated mass of grey matter, which 

 lies in the upper half of the pons ; further, some cells which lie 

 in the grey matter around the cerebral aqueduct (of Sylvius) in 

 the mid-brain send their fibres down to join the motor root of 

 the trigeminal. The nucleus receives fibres from the cortex 

 of both cerebral hemispheres, and, therefore, supra-nuclear 

 lesions do not cause complete paralysis of the muscles supplied 

 on the opposite side of the body. Nuclear lesions are accom- 

 panied by complete paralysis of the muscles of mastication 

 (p. 70) on the same side as the lesion ; they sometimes occur 

 late in the course of bulbar paralysis (p. 108). 



The SENSORY NUCLEUS is placed in the lower half of the 

 pons, and extends downwards through the whole length of the 

 medulla oblongata into the spinal medulla, in which it reaches 

 the level of the second cervical segment. In the pons, it lies 

 postero-lateral to the nucleus of the facial nerve and lateral to 

 the ascending sensory fibres (spino-thalamic tract) from the 

 spinal medulla (Fig. 34). A nuclear lesion, therefore, not only 

 causes anaesthesia in the area of distribution of the trigeminal, 

 but it may also cause partial or complete anaesthesia of the 

 limbs and trunk on the opposite side, owing to involvement 



