SPINAL ACCESSORY. 475 



into two principal branches: First the internal or anastomotic 

 branch, which joins the pneumogastric nerve, and becomes mingled 

 with its fibres; and, secondly, the external or muscular branch, 

 which passes downward and outward/and is distributed to the 

 sterno-mastoid and trapezius muscles. 



The spinal accessory is essentially a motor nerve. It has been 

 found, both by Bernard and Longet, to be insensible at its origin, 

 like the anterior roots of the spinal nerves ; but if irritated after 

 its exit from the skull, it gives signs of sensibility. This sensibi- 

 lity it acquires from the filaments of inosculation which it receives 

 from the anterior branches of the first and second cervical nerves. 

 Though its external branch, accordingly, is exclusively distributed 

 to muscles, as we have already seen, this branch contains some sensi- 

 tive fibres, which have the same destination. The reason for this 

 anatomical fact, viz., that motor nerves are supplied during their 

 course with sensitive fibres, becomes evident when we reflect that the 

 muscles themselves possess a certain degree of sensibility, though 

 less acute than that which belongs to the skin. The sensibility of 

 the muscles is undoubtedly essential to the perfect performance of 

 their function ; and as the motor nerves are incapable, by them- 

 selves, of transmitting sensitive impressions, they are joined, soon 

 after their origin, by other filaments which communicate to them 

 this necessary power. 



The most important result which has been obtained by experi- 

 ment upon the spinal accessory nerve, is that its internal or anasto- 

 motic branch is directly connected with the vocal movements of the 

 glottis. It has been found by Bischoff, by Longet, and by Bernard, 

 that if the spinal accessory nerves on both sides, or their branches 

 of inosculation with the pneumogastric, be divided or lacerated, 

 the pneumogastric nerves themselves being left entire, the voice is 

 instantly lost, and the animal becomes incapable of making a vocal 

 sound. We have also found this result to follow, in the cat, after 

 the spinal accessory nerves have been torn out by their roots, 

 through the jugular foramen. The animal, after this operation, can 

 no longer make an audible sound. At the same time the respira- 

 tory movements of the glottis go on undisturbed, and most of the 

 other animal functions remain unaffected. 



The fibres of communication, therefore, derived from the spinal 

 accessory, pass to the pneumogastric nerve and become entangled 

 with its other filaments, so that they can no longer be traced by 

 anatomical dissection. They pass downward, however, and become 



