io 4 THE NERVOUS SYSTEM 



NUCLEAR LESIONS occur in the late stages of progressive 

 muscular atrophy or as the result of a downward spread in 

 bulbar paralysis (p. 108). Since the lesion is bilateral, both the 

 sterno-mastoids and both the trapezius muscles are paralysed. 

 The head therefore falls forward and cannot be extended on 

 the trunk. This condition, however, is not wholly referable 

 to the trapezius, as the deeper muscles at the back of the 

 neck are also involved. 



INFRA-NUCLEAR LESIONS of the accessory nerve may occur 

 (a) in the anterior triangle of the neck, and (b) in the posterior 

 triangle. 



(a) Neuritis of the accessory gives rise to the condition of 

 spasmodic torticollis, which is due to spasmodic contraction 

 of the sterno-mastoid, on the side of the lesion. This 

 condition may be accompanied by spasm of the upper fibres 

 of the trapezius, resulting in spasmodic elevations of the point 

 of the shoulder, which synchronise with the torticollis. 



Paralysis of the sterno-mastoid is marked by tonic torti- 

 collis, which is not always pronounced, but in this case the 

 condition is due to the tonus of the unopposed muscle of 

 the sound side. 



When the trapezius is paralysed, the point of the shoulder on 

 the affected side drops to a lower level, because, under normal 

 conditions, the upper part of the muscle helps to support the 

 weight of the upper limb. In addition, since the lower neurone 

 is affected, the muscles atrophy and exhibit alterations in their 

 electrical excitability. Owing to atrophy of the trapezius, the 

 normal rounded contour of the side of the neck is lost and 

 the resulting " square " appearance is quite characteristic. 



(b) When the accessory nerve is injured in its course across 

 the posterior triangle, only the trapezius is affected. As the 

 injury is commonly caused by heavy weights, carried on the 

 shoulder, the long thoracic nerve (of Belt] may also be involved 

 and the deformity is more complicated (p. 133). 



The Hypoglossal or Twelfth Cerebral Nerve is purely 



