THE REFLEX ARC 



253 



all the way to the posterior end of the medulla oblongata. 

 In this region they connect with the dendrites of neurons, 

 whose axons form a ventral commissure across the cord 



Ventral commissure 



Skirt 



Sensory tract 



Motor track^ 

 Motor root-.. 



Sensory rooL.. / 



FIG. 70. Diagrammatic dorsal view of the upper end of the spinal 

 cord of larval Amblystoma, illustrating the relations of the giant cells, 

 commissural cells, and motor cells and the mechanism of the swimming 

 reflex (based on the researches of Coghill, 1913, 1914; from Herrick 

 and Coghill, 1915). 



and connect with the motor neurons of the opposite 

 side. These motor neurons send axons down the cord 

 and collaterals to the muscles. At first the commissures 

 occur only at the posterior end of the medulla, but 

 in later stages the region of commissural connection 



