THE SPINAL CORD. 



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ties of the brain so carefully as does the pia mater. Thirdly, a firm 

 membrane— the dura mater— lines the brain case, and is continued down 

 the spmal canal. In lower Vertebrates the dura mater is double, in 

 higher Vertebrates it is so in the region of the spinal cord, where the 

 outer part lines the bony tunnel, while the inner ensheathes the cord 

 itself. In Fishes the brain case is much larger than the brain, and a 

 large lymph space lies between the dura and the pia mater. 



An understanding of the relations of the different regions will be 

 facilitated by a study of the following table which Dr. Gadow gives in 

 his great work on Birds in Bronn's Thierreich : — 



The Spinal Cord. 



After the formation of the brain vesicles, the remainder of 

 the medullary canal forms the spinal cord. 



The canal is for a time continuous posteriorly with the 

 food canal beneath, so that a n -shaped tube results. The 

 connection between them is called the neurenteric canal, 

 and though it is only temporary its frequent occurrence is of 

 much interest. 



The wall of the medullary canal becomes very much 



