THE VISCERAL EFFERENT DIVISION. 203. 



cord and the roots take their exit from the dorso-lateral surface of 

 the cord between the dorsal and ventral roots of the spinal nerves, 

 run forward along the side of the medulla oblongata and join 

 the trunk of the vagus (Figs. 20, 32). These roots have the name 

 of the spinal accessory or XI cranial nerve. In mammals the 

 nucleus and roots of this nerve extend farther caudad than in 

 other classes, are more variable in position and show a greater 

 tendency to a segmental arrangement and a closer relation with 

 the dorsal roots of the spinal nerves. The more caudal roots 

 are smaller, more nearly segmental and are placed nearer the 

 dorsal roots (I.ubosch). It seems that above the amphibia the 

 increasing importance of the trapezius muscles have been corre- 

 lated with an increase in the extent of the accessorius nucleus. 

 The fact that skeletal muscles important in the movements 

 of the fore limb are innervated by nerves arising from the visceral 

 motor column requires a word of explanation. All other muscles 

 involved in general bodily movements are derived from the dorsal 

 mesoderm and are innervated by nerves from the ventral motor 

 column. A somewhat similar anomaly is seen in the muscles 

 of mastication. Although these are derived from the lateral 

 mesoderm and are innervated by visceral motor fibers they are 

 voluntary muscles which move skeletal parts whose functions are 

 much more than merely visceral functions. Although primarily 

 all muscles derived from the lateral mesoderm may have been 

 related solely to the walls of the alimentary canal and have been 

 involuntary in their action, it is evident that neither of these 

 characters are retained by all such muscles. What is found to be 

 constant is that muscles derived from the lateral mesoderm are 

 innervated by nerves arising from the visceral motor column. 

 The chief question regarding the trapezius musculature, then, is 

 how it comes to be attached to the skeleton of the arm. The 

 only probable explanation is that the shoulder girdle or pectoral 

 arch did not have its origin as a part of the skeleton of a limb, 

 but existed as a branchial arch before the limb was formed. It 

 is believed that primitive vertebrates possessed a considerably 

 larger number of gills than are now found in most vertebrates 

 and it is supposed that the skeleton of one or more branchial 



