1274 



HUMAN ANATOMY. 



its irritation causes spasm of the laryngeal muscles, with brassy cough and stridulous 

 breathing. The tendency to closure of the glottis is sometimes so threatening 

 as to demand immediate tracheotomy or intubation. Paralysis causes hoarseness 

 or loss of voice (aphonia). In a bilateral paralysis both cords fall into the cadaveric 

 position. Loss of voice results and marked inspiratory dyspnoea, which may demand 

 tracheotomy or intubation. 



THE SPINAL ACCESSORY NERVE. 



The eleventh or spinal accessory nerve (n. accessorius) is purely motor. It con- 

 sists of two portions, a spinal and an accessory, which differ widely in origin, course 

 and distribution. The spinal portion or accessories spinalis (r. externus) is so termed 

 because it arises from the spinal cord and the accessory portion or accessorius vagi 

 (r. internus) receives its name in recognition of the fact that it is accessory to the 

 vagus. As emphasized in connection with the last-named nerve (page 1266), the 

 so-called accessory portion of the eleventh is, in reality, an integral part of the vagus 

 and the description of its deep origin and distribution has been included with those 

 of the vagus. There remains, therefore, only the spinal portion of the nerve to be 

 considered. The spinal part — the eleventh nerve proper — supplies the sterno- 

 mastoid and trapezius muscles. 



Deep Origin. — The fibres constituting the spinal part of the nerve arise as the 

 axones of a column of large multipolar neurones which is situated in the anterior 

 horn of the spinal gray matter and extends from the lower end of the medulla to 

 the fifth or sixth cervical segment of the spinal cord. The cells of this column, 

 known as the accessory nucletis, occupy a dorso-lateral position in the horn, lying 

 posterior to the cells from which arise the fibres of the anterior roots of the cervical 

 nerves. Leaving these cells, the fibres pass dorsally within the gray matter to the 

 vicinity of the bay between the anterior and posterior horns, where, while some at 

 once curve outward and traverse the white matter to gain the lateral surface of the 

 cord, the majority bend abruptly brainward and pursue a short ascending path before 

 turning outward. 



Course and Distribution. — The superficial origin of the accessory nerve 

 is marked by the emergence of a series of fasciculi along the lateral surface of the 

 spinal cord between the anterior and posterior roots of the cervical spinal nerves, the 

 fasciculi progressively nearing the posterior roots as they issue at higher levels. 

 Consecutively joining shortly after they escape from the cord, the fasciculi unite to 

 form a common trunk, which gradually increases in size by accessions of fibres at 

 each succeeding segment. The nerve-trunk thus formed passes upward in the sub- 

 dural space, between the ligamentum denticulatum and the posterior nerve-roots 

 (Fig. 879), to the foramen magnum, through which it enters the cranium. Upon 

 reaching the side of the medulla, the spinal accessory nerve turns outward to enter 

 the middle compartment of the jugular foramen and to unite temporarily with the 

 accessory vagus. It occupies the posterior part of the middle compartment of the 

 jugular foramen, lying within a dural sheath which contains also the vagus. On 

 reaching the lower margin of the foramen, the fibres accessory to the vagus perma- 

 nently leave the eleventh nerve. The latter, often described as the spinal part, 

 courses downward for a short distance in the interval between the internal carotid 

 artery and the internal jugular vein and then passes backward, either anterior or pos- 

 terior to the vein, until it reaches the deep surface of the sterno-mastoid muscle, 

 which it usually enters. While within the substance of the muscle, the spinal 

 accessory gives off filaments which unite with a branch from the second cervical 

 n&v\& \.oiorm the sterno-mastoid plexus (¥\g. 1082) for the supply of that muscle. 

 Emerging from beneath the posterior edge of the sterno-mastoid, the eleventh 

 nerve crosses the occipital triangle and dips under the anterior margin of the 

 trapezius along the deep surface of which it descends almost to the lower margin 

 of the muscle. Under the trapezius the nerve forms a plexus of varying degrees 

 of intricacy with the third and fourth cervical nerves. This is called the 

 uibtrapezial plexus ( Fig. 1082 ), its fibres of distribution supplying solely the 

 trapezius muscle. 



