THE NERVOUS SYSTEM 



Longitudinal Nuclear Columns. In a previous chapter we learned that at 

 an early stage in its development the lateral wall of the neural tube consists of a 

 dorsal or alar and a ventral or basal plate, separated by a groove, the sulcus 

 limitans (Fig. 119). The sensory nuclei of the cranial nerves develop within the 

 alar plate and the motor nuclei within the basal plate. In the rhombencephalon 

 both plates come to lie in the floor of the fourth ventricle, the alar occupying 

 the more lateral position. And, in spite of the changes of position which occur 

 during development, the sensory nuclei retain, on the whole, a lateral, and the 

 motor nuclei a more medial, location. From the basal plate there differentiate 

 a somatic and a visceral column of efferent nuclei, and from the alar plate a 

 visceral and a somatic column of afferent nuclei. 



The somatic e/erent column includes the nuclei of those motor nerves which 

 supply the striated musculature derived from the myo tomes, i.e., the extrinsic 

 muscles of the eye and the musculature of the tongue (Figs. 119-121). 



The visceral efferent column undergoes subdivision into: (1) a ventrolateral 

 column of nuclei, from which arise the special visceral efferent fibers to the striated 

 visceral or branchial musculature, and which includes the nucleus ambiguus and 

 the motor nuclei of the fifth and seventh nerves; and (2) a more dorsally placed 

 group for the innervation of involuntary musculature and glandular tissue, of 

 which the dorsal motor nucleus of the vagus is the chief example. The former 

 may be called the special visceral efferent and the latter the general visceral ef- 

 ferent column. 



The visceral afferent column is represented by the nucleus of the tractus 

 solitarius, within which end the afferent fibers from the visceral mucous membrane 

 and the taste buds, i. e., both the general and special visceral afferent fibers. 

 The somatic afferent column splits into two: a general somatic afferent column, 

 within which terminate the sensory fibers from the skin; and a special somatic 

 group of nuclei for the reception of the fibers of the acoustic nerve and, in aquatic 

 vertebrates, of the lateral line nerves also. 



THE SOMATIC EFFERENT COLUMN 



As can be seen by reference to Figs. 101, 108, 114, and 116 the nuclei of the 

 hypoglossal, abducens, trochlear, and oculomotor nerves are arranged in linear 

 order in the central gray matter near the median plane. They represent the 

 continuation into the medulla oblongata of the large cells of the anterior column 

 of the spinal cord. The cells of these nuclei are large and multipolar with 

 well-developed Nissl bodies (Fig. 126). From them arise large myelinated 



