174 THE NERVOUS SYSTEM 



are assembled into bundles which run ventrally toward their exit along the 

 lateral border of the pyramid. 



A conspicuous plexus of myelinated fibers gives the hypoglossal nucleus a 

 characteristic appearance in Weigert preparations. Fibers from many sources 

 reach the nucleus and ramify within it. These include some from the cortico- 

 bulbar tract and others from the sensory nuclei of the fifth nerve and from the 

 nucleus of the tractus solitarius. The part which such fibers may play in reflex 

 movements of the tongue is illustrated in Fig. 92. 



THE SPECIAL VISCERAL EFFERENT COLUMN 



The special visceral efferent column of nuclei contains the cells of origin of 

 the motor fibers for the striated musculature derived from the branchial arches, 

 as distinguished from the general skeletal musculature that develops from 

 the myotomes. The branchial musculature includes the following groups of 

 muscles: the muscles of mastication, derived from the mesoderm of the first 

 branchial arch and innervated by the trigeminal nerve; the muscles of expression, 

 derived from the second or hyoid arch and innervated by the facial nerve; the 

 musculature of the pharnyx and larynx, derived from the third and fourth arches 

 and innervated by the glossopharyngeal, vagus, and accessory nerves; and prob- 

 ably also the sternocleidomastoid and trapezius muscles, innervated through the 

 spinal root of the accessory nerve. Some authors prefer to call this column, 

 which includes the motor nuclei of the fifth and seventh nerves and the nucleus 

 ambiguus, the lateral somatic column, because the cells in these nuclei and the 

 fibers which arise from them possess the characteristics of somatic motor cells 

 and fibers (Malone, 1913). The nuclei are composed of large multipolar cells 

 with well-developed Nissl bodies. These cells give origin to large myelinated 

 fibers which run through the corresponding nerve and terminate in neuromus- 

 cular endings in one or another of the muscles indicated above. 



The motor nuclei of the fifth and seventh nerves and the nucleus ambiguus 

 of the ninth, tenth, and eleventh nerves form a broken column of gray matter, 

 located in the ventrolateral part of the reticular formation of the pons and 

 medulla oblongata some distance beneath the floor of the fourth ventricle (Figs. 

 121, 122). The cells of this column and the special visceral efferent fibers which 

 arise from them have been colored blue in Figs. 119 and 120. 



The motor nucleus of the trigeminal nerve lies on the medial side of the 

 main sensory nucleus of that nerve, and is located at the level of the middle 

 of the pons in the lateral part of the reticular formation some distance from the 



