THE NERVOUS SYSTEM. 



491 



proceed to the visceral receptors. These latter fibers are thus visceral afferent 

 fibers. 



It is now known that the spinal ganglion is a much more complicated struc- 

 ture and has more forms of nerve cells than was formerly realized. The dif- 

 ferentiation into these various types has not yet been fully observed. The 

 bipolar cells, however, become unipolar in the manner shown in Fig. 431. 

 The cell body first becomes eccentrically placed with reference to the two proc- 

 esses and then, as it were, retracts from them, remaining connected with them 

 by a single process. This change may economize space. 



According to most authorities, many of the cells of the neural crest do not 

 cease their migration by forming spinal ganglia, but undifferentiated cells 



FIG. 431. Section of spinal ganglion of 1 2-day chick embryo. Cajal. 



Showing various stages of the change from the bipolar to the unipolar condition. A,B, Unipolar 

 cells; C, D, F, G, cells in transitional stage; E, bipolar cell; H, immature cell. The neuro- 

 fibrils are well shown. 



wander still further ventralward and form, probably also undergoing still 

 further proliferation, the rudiments of the various sympathetic ganglia, becom- 

 ing subsequently differentiated into the sympathetic cells. By this migration 

 there is first formed a longitudinal column of cells ventral to the spinal ganglia 

 (Fig. 433) and, later, in relation with the white communicating rami (Fig. 

 432). This column becomes segmented (seventh week), forming ultimately 

 the ganglia of the vertebral sympathetic chain. In the meanwhile, the 

 cells of the column proliferate in places, forming rudiments which, by migra- 

 tion and further differentiation, form the ganglia of the various prevertebral 

 sympathetic plexuses (cardiac, cceliac, pelvic, etc.). Further migrations lead to 

 the formation of the ganglia of the peripheral plexuses (Auerbach, Meissner, 



