488 STRUCTURE OF VERTEBRATA. 



Enswathing the brain and spinal cord, and following its irregularities, 

 is a delicate membrane the pia mater rich in blood vessels, which 

 supply the nervous system. Outside this, in higher Vertebrates, there 

 is another membrane the arachnoid which does not follow the minor 

 irregularities 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 spinal canal. In lower Vertebrates the dura 

 mater is double throughout ; in higher Vertebrates it is double only in 

 the region of the spinal cord, where the outer part lines the bony 

 tunnel, while the inner ensheaths 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 : 



