THE TRIGEM1NAL NERVE 73 



as an elevator of the hyoid bone than as a depressor of the 

 mandible. The posterior belly (facial nerve, p. 81) arises 

 on the medial side of the base of the mastoid process and 

 passes forwards and downwards, deep to the angle of the 

 mandible, to the greater cornu of the hyoid bone. It 

 terminates in the common tendon of the muscle, which is 

 attached to the greater cornu by a slip of the deep cervical 

 fascia. The anterior belly passes forwards and medially 

 from the common tendon to be attached to the base of the 

 mandible near the median plane. It is placed superficial to 

 the mylo-hyoid, but is partially overlapped by the submaxillary 

 salivary gland. 



Paralysis of the motor part of the trigeminal is usually 

 accompanied by some involvement of the sensory fibres. 

 Owing to bilateral representation in the cerebral cortex (p. 

 34), the muscles of mastication are rarely affected in lesions 

 of the internal capsule or the corona radiata. 



Nuclear lesions have already been referred to (p. 61). 



The motor root of the fifth, as it lies on the apex of the 

 petrous part of the temporal bone, may be affected in 

 syphilitic basal meningitis, and, in these cases, the whole of 

 the sensory distribution of the trigeminal will be involved, 

 probably together with the third and fourth cerebral nerves. 



The motor root may be affected in extra-cranial tumours 

 which compress the mandibular nerve. 



In complete motor paralysis of the trigeminal th'e principal 

 disability is caused by the lack of opposition to the pterygoid 

 muscles of the sound side. The unparalysed elevators and 

 depressors are sufficiently powerful to carry out their respective 

 movements, but the tonus of the unopposed pterygoids causes 

 the jaw to be thrust over to the paralysed side, and so the 

 teeth do not oppose one another in a satisfactory manner 

 during mastication. 



Some authorities state that the free border of the soft 

 palate lies at a lower level on the paralysed side, but others 

 have failed to observe any affection either of the soft palate 



