The Spinal Accessory Nerve and Its Musculature

Abstract

T HE fact that in mammals the socalled accessory field of musculature, comprising the trapezius and sternocleidomastoid muscles, is innervated both by ramus 'lateralis n. accessorii and by branches of the anterior rami of the cervical nerves, is a circumstance that has never ceased to puzzle anatomists. The present writers have been interested in this problem for a number of years and have borne it constantly in mind during the course of their investigations. At present it is almost universally believed that ramus lateralis n. accessorii is of visceral derivation. Recently, however, Addens (I933) has revived the old claim (see discussions by Fiurbringer, I897, and Lubosch, I899) that this nerve is a spinal component of somatic origin. Addens bases his contention on three main points: (a) the presence in a few fish of a component of n. X which he interpreted as somatic and the homologue of spinal n. XI; (b) the apparent continuity of the spinal XI nucleus with that of n. XII in some forms; and (c) the assumption of some investigators that m. trapezius is of myomeric rather than branchiomeric origin. Addens thus believes that the spinal accessory nerve and trapezius sheet of musculature have somatic rather than branchiomeric genesis, a contention requiring the thesis that in this respect mammals are more primitive than the dogfish. If ramus lateralis n. accessorii be merely a modified somatic spinal component, then the dual innervation of the spinal accessory field of musculature involves no particular problem. On the other hand, if this nerve be of visceral derivation, the double innervation of its musculature is myologically unique. In such case either the muscles are of compound visceralsomatic origin, or else, primarily visceral, some of their nerve components have secondarily shifted to follow a somatic (spinal) pathway.