280 



STRUCTURE OF THE VERTEBRATES 



end at the spinal motor nerve cells. An examination of the cord 

 sectioned through the two enlargements will show that there 

 is more white than gray substance in the cervical region, and 

 that the arrangement is exactly reversed in the sacral en- 

 largement. 



In a general way, the fibers of the cord which carry similar 



Brain 



Dorsal root 



Dorsal ganglion 



Sensory neurone 



Fig. 159. Sections of the Cord at Different Levels. Sensory fibers are added 



at each spinal nerve, and motor fibers decrease in number at each more 



posterior level. Observe that the sensory 'tracts are concentrated in the 



dorsal region, and the motor tracts are mainly in the ventral half. 



