TRANSFORMATION OF NEURAL TUBE INTO SPINAL CORD 



163 



Finally, the differentiation of the gray matter results in dorsal and ventral 

 horns. These contain the cell bodies for the nerve fibers which form the white 

 matter of the spinal cord. The central canal becomes very small in diameter 

 but it never disappears and it persists throughout adult life. It contains the 

 cerebrospinal fluid. 



While differentiation is going on within the spinal cord, the adjacent 

 mesenchyme migrates over the surface of both the notochord and the spinal 

 cord and soon envelops these structures with a supporting tissue. This 

 mesenchyme first forms cartilage, which is finally replaced by bone of the 

 vertebrae. 



The over-all picture of the relationships between the various elements of 

 the nervous system is shown in Figure 96. The spinal ganglia connect with 



GANGLION 



SPINAL GANGLION 



SECONDARY 



SYMPATHETIC 



GANGLION 



PRIMARY 



SYMPATHETIC 



GANGLION 



Fig. 96. The relationships of the nerve cord, the spinal ganglia, the sympa- 

 thetic ganglia, and the viscera. The viscera receive two separate sets of nerves. 

 One set arises in the spinal cord and, by way of the sympathetic ganglia and 

 the deeper ganglia, supply the viscera. Another set arises in a cranial ganglion 

 and runs through the vagus nerve to the viscera. Thus a dual control of the 

 viscera is achieved by a dual innervation. 



