THE CRANIAL NERVES AND THEIR NUCLEI 177 



emerge from the brain (Fig. 105). A few of the fibers cross the median plane 

 and join the corresponding root bundles of the opposite side. 



The accessory nerve consists of a bulbar and a spinal portion. The fibers of the spinal 

 root take origin from a linear group of cells in the lateral part of the anterior gray column 

 in the upper cervical segments of the spinal cord. This root ascends along the side of the 

 spinal cord, passes through the foramen magnum, and is joined by the bulbar rootlets of the 

 accessory (Fig. 125). The nerve then divides into an internal and an external branch. In 

 the latter run all the fibers of spinal origin and these are distributed to the trapezius and 

 sternocleidomastoid muscles. If, as seems probable, these muscles are derived from the 

 branchial arches (Lewis, 1910), the fibers which supply them may be regarded as special 

 visceral efferent fibers; and the spinal nucleus of the accessory nerve may be considered as 

 homologous to the nucleus ambiguus. The bulbar rootlets of the accessory nerve, which con- 

 tain both general and special visceral efferent fibers, form a well-defined fascicle, readily 

 distinguished from the spinal portion of the nerve, which, as the internal ramus, joins (he 

 vagus nerve and is distributed through its branches (Fig. 120 Chase and Ranson, 1914). 



The sensory collaterals which arborize among the cells of the nucleus am- 

 biguus are derived from the central tracts of the trigeminal, glossopharyngeal, 

 and vagus nerves, from ascending sensory fibers of spinal origin, and from other 

 longitudinal fibers in the reticular formation. Other fibers reach this nucleus 

 from the corticobulbar tract. 



THE GENERAL VISCERAL EFFERENT COLUMN 



The general visceral efferent column of nuclei is composed of the cells from 

 which arise the efferent fibers innervating cardiac and smooth muscle and glan- 



- ' O^S^A * iaS 

 JP 



A B 



Fig. 126. Two types of motor nerve-cells from medulla oblongata of lemur: A, Cells of the 

 somatic motor type from the hypoglossal nucleus; B, cells of the visceral efferent type from the 

 rostral part of the dorsal motor nucleus of the vagus. Toluidin blue stain. (Malone.) 



dular tissue. The cells of these nuclei are of small or medium size and their 

 Nissl bodies are not well developed (Fig. 126). They give rise to the general 



