CHAPTER V. 



MEMBRANES OF THE SPINAL CORD. 

 (MENINGES SPINALIS.) 



Dura Mater. Through the foramen magnum the membranes 

 of the brain are continuous with those of the cord with which they 

 are very similar in structure. The dura mater spinalis is 'attached 

 to the margin of the great foramen and to the bodies of the first 

 two or three cervical vertebrae; elsewhere, though joined to the 

 vertebrae by fibrous bands, its surface is free from immediate bony 

 attachment and it does not possess the periosteal layer. Thus 

 suspended, it hangs as an open sack, or sheath (Fig. 100) and 

 reaches down to the third sacral vertebra, where it is constricted 

 to a fibrous cord which blends with the periosteum on the posterior 

 surface of the coccyx. The arachnoid and pia, and the spinal 

 cord and cauda equina are contained in the dural sack (Figs. 

 97, 98 and 100). Externally, the surface of the dura is separated 

 from the wall of the spinal canal by the internal vertebral plexus 

 of veins, areolar tissue and fat. The outer surface is composed 

 of flat polygonal cells, like the inner surface. Its internal, serous 

 surface is bathed with a small amount of cerebro-spinal fluid which 

 separates it from the arachnoid. For every segment of the spinal 

 cord, the dura presents, on either side, a pair of foramina, through 

 which run the anterior and posterior roots of the spinal nerves 

 (Fig. 97). Those nerve roots are invested by a sheath of dura 

 prolonged from the margins of the foramina. The dura mater 

 of the cord does not separate into two layers, and forms neither 

 sinuses nor processes. It performs no periosteal function and 

 possesses no arachnoid granulations (pacchionian bodies). Its 

 two surfaces are formed by endothelium. 



Arachnoid. The arachnoid of the spinal cord (arachnoidea 

 spinalis) forms a sack of the same length as the dural sheath, 

 with which it is externally in contact (Figs. 97, 98 and 100). It 



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