712 



ACCESSORY NERVE OF WILLIS. 



XI. ACCESSORY NERVE OF WILLIS. 



The single elongated nucleus of origin (Fig. 241) comprises the dorsolateral 

 group of cells of the anterior horn of the cervical cord, which begins below at the 

 level of the seventh cervical nerve and extends upward without interruption in the 

 medulla oblongata to the upper extremity of the pyramidal decussation. The 

 nucleus of origin approaches at its highest point the hypoglossal nucleus, then 

 is situated above the first cervical nerve in the middle of the anterior horn, 

 next passes laterally, and between the second and fourth nerves is situated at 

 the lateral margin of the anterior horn. Still further downward, to below the 

 sixth cervical nerve, it is situated at the base of the lateral horn. All fibers 

 arise from the ganglia as neurites. From the cortical center on the opposite side 

 there must pass to the nucleus fibers through which voluntary stimulation of the 

 motor fibers is effected. 



The fibers pass upward in the lateral column of the spinal cord and leave the 

 latter in several bundles between the anterior and posterior cervical nerve-roots. 

 Then the root-fibers that ascend through the great occipital foramen come together 

 without uniting in the neighborhood of the jugular foramen and form the two 

 branches of the nerve. Of the latter the inner enters wholly into the gangliform 

 plexus (Fig. 246) and supplies the vagus with most of its motor fibers and also 

 its cardiac inhibitory fibers. In man, accordingly, total paralysis of the accessory 

 nerve is attended with immobility of the corresponding half of the larynx and 

 soft palate. 



According to Kreidl the inhibitory fibers for the heart are situated in the 

 most anterior root-bundles of the inner branch of the accessory nerve. If these 

 roots are divided, the cardiac inhibitory fibers undergo degeneration. If the 

 trunk of the vagus in the neck is irritated four or five days after the operation the 

 cardiac inhibitory action is no longer exhibited. 



The external branch of the accessory nerve is derived from the spinal 

 portion of the nucleus. This anastomoses with sensory filaments from 

 the posterior root of the first, less commonly also of the second cervical 

 nerve, which supply muscle-sense fibers to it. It then passes backward 

 over the transverse process of the atlas and terminates as a motor nerve 

 in the sternocleidomastoid and trapezius muscles (Fig. 246). The latter 

 large muscle receives, however, motor filaments for its acromial portion 

 from the cervical plexus. 



The external branch anastomoses also with several cervical nerves. Either 

 these fibers take part in the innervation of the muscles named, or the accessory 

 nerve returns to them, in part, the sensory filaments received from the posterior 

 roots of the two uppermost cervical nerves, which then constitute the cuta- 

 neous branches of these cervical nerves. 



Pathological. Irritation of the external branch causes clonic and tonic spasm 

 of the muscles named, usually upon one side. If the branch for the sternocleido- 

 mastoid is alone affected, the head responds to the traction of this muscle in 

 the presence of clonic spasm. If the disorder is bilateral, the traction is usually 

 alternating; much less commonly the action is bilateral; so that the head executes 

 a nodding movement. In the presence of clonic spasm of the trapezius, the head 

 is drawn backward and to the side; the scapula generally follows the traction of 

 the bundle of this great muscle that is most severely involved. Tonic spasm of 

 the sternomastoid causes the characteristic position of caput obstipum (spasticum) 

 -spasmodic wry-neck. Similar spasm of the trapezius usually involves only indi- 

 vidual portions of the muscle, which then naturally cause special positions of the 

 head or of the scapula. Irritation of the root causes at the same time spas- 

 modic movements of the muscles of the larynx and of the uvula. f 



Paralysis of one sternomastoid causes the head to be turned toward the opposite 

 side by the preponderant action of the muscle of that side (paralytic torticollis} . 

 Paralysis of the trapezius is usually confined to individual portions of the muscle. 

 Paralysis of the entire accessory trunk, principally in consequence of central pro- 

 cesses, gives rise, in addition to paralysis of the sternocleidomastoid and the tra- 

 pezius, also to paralysis of the motor branches of the vagus previously mentioned. 

 Bilateral paralysis is extremely rare and is said to be attended with acceleration 

 of the heart-beat. 



