294 George L. Streeter 



ganglion of the trunk, the latter being temporarily connected with the 

 placode over the third arch. As can be seen from the relative size of 

 the ganglia the nerve consists mostly of sensory fibres, connected periph- 

 erally with the structures developing from the second (r. tympanicus) 

 and third (r. lingualis) arches. The tympanic branch is not well defined 

 until we come to embryos between twelve and fourteen mm. long. Cen- 

 trally the rootlets enter the brain wall and, joining with the fibres from 

 the facial, extend caudally (Plate III) as the tractus solitarius. The 

 motor rootlets of this nerve arise from a group of neuroblasts in the 

 nucleus ambiguus series, situated beneath the floor of the fifth rhombic 

 groove. The motor bundles extend directly lateral beneath this groove 

 and pass under the spinal tract of the trigeminal and then emerge from 

 the brain wall and join the main trunk of the nerve. 



The n. vagus like the facial represents a branchial nerve the motor 

 fibres of which have in man undergone special development, for the pur- 

 pose of supplying a group of muscles derived from its branchial arch 

 or arches. The large motor trunk of the facial nerve, as we have seen, 

 is distributed to the muscle cells of the hyoid arch and, as these cells 

 group themselves into the muscles of expression and spread forward over 

 the face, the facial branches are drawn along with them. In a similar 

 way the more caudal rootlets of the vagus become predominantly motor 

 and form a distinct bundle which we know as the spinal accessory nerve, 

 and this bundle is distributed to a group of muscle cells originally be- 

 longing to the more caudal branchial arches, and in man are destined 

 to form muscles for the arm girdle, the mm. sternocleidomastoideus and 

 trapezius. As these muscles spread out into their eventual position 

 the nerve is drawn down across the neck with them. Coincident with 

 the increased importance of this musculature as we ascend the vertebrate 

 scale we meet with increased development of the spinal accessory, and 

 it obtains additional rootlets of origin by spreading down into the region 

 of the spinal cord. As can be seen in Plate I, the spinal accessory may 

 reach as far down as the fourth cervical segment. The nucleus of origin 

 of the spinal accessory and other motor rootlets of the vagus constitutes 

 the nucleus ambiguus of the medulla oblongata and the lateral horn of 

 the spinal cord, the two being directly continuous. This is best shown 

 in Plate II. 



The neuroblasts of the basal plate of the neural tube form two columns, 

 a larger median one and a smaller lateral one ; the median column 

 furnishes somatic motor fibres to the ventral spinal roots and hypoglossal,^ 



