THE ACCESSORY NERVE 103 



lateral aspect of the spinal medulla, midway between the 

 anterior and posterior roots of the upper five cervical nerves. 

 They ascend in the vertebral canal, forming a common trunk, 

 which enters the cranium through the foramen magnum and 

 passes to the jugular foramen. Outside the skull the spinal 

 portion of the accessory runs downwards, backwards and 

 laterally through the neck (Fig. 49), and it is entirely distributed 

 to the sterno-mastoid and the upper part of the trapezius. 



The sterno-mastoid arises from the manubrium sterni and 

 the medial third of the clavicle and passes upwards, backwards 

 and laterally to be inserted into the mastoid process and the 

 occipital bone. Contraction of the muscle approximates its 

 insertion to its origin, and therefore the mastoid process is 

 approximated to the manubrium, i.e. the head is rotated 

 towards the opposite side and, at the same time, the chin is 

 tilted upwards. 



The upper part of the trapezius arises from the external 

 occipital protuberance and the ligamentum nuchae, and its 

 fibres pass downwards and laterally to be inserted into the 

 lateral third of the clavicle. When the muscle contracts, it 

 elevates the point of the shoulder and, in association with 

 the serratus anterior (s. magnus), rotates the scapula clock- 

 wise (as viewed from in front), enabling the arm to be flexed 

 and abducted beyond a right angle (p. 132). Further, when 

 the body is in the erect or sitting posture with the arms 

 unsupported, the weight of the upper limb is partially borne 

 by the upper portion of the trapezius. 



SUPRA-NUCLEAR LESIONS of the fibres of the spinal portion 

 of the accessory nerve are never isolated, and occur most 

 commonly in cerebral hemiplegia in company with extensive 

 paralysis of the limb muscles. The skrno-mastoid, though 

 weakened, is not paralysed, since it is innervated from the 

 cortex of both cerebral hemispheres. The upper part of the 

 trapezius is not completely paralysed, but the point of the 

 shoulder, being depressed by the weight of the limb, occupies 

 a lower level than the point of the sound shoulder. 



