THE NERVOUS SYSTEM 127 



So close does the hypophysis lie to the infundibulum that Miss 

 O'Neill states she is unable to detect the neural layer of the dura 

 mater between them, and therefore suspects that at this point the 

 interdural space and the subdural space may actually communicate 

 with one another. 



(iv) Spatium interdurale paraphyseos (O'Neill). The fourth inter- 

 dural space is a median dorsal one. The paraphysis is a dorsal out- 

 growth from the roof of the thalamencephalon (see under 'Brain'), 

 and as it comes into contact with the dura mater during its develop- 

 ment, it penetrates the neural layer and spreads out between this 

 layer and the periosteal layer, thus producing a space between them. 

 It will be noticed that this space is produced in exactly the same 

 manner as those already described, but in the reverse direction, 

 namely, by a centrifugal outgrowth instead of a centripetal one. 

 Since the paraphysis remains attached to the brain, the spatium 

 interdurale paraphyseos retains a communication with the subdural 

 space. 



The primary vascular coat of the brain entirely envelops this im- 

 portant organ. It takes part in the formation of the several plexuses 

 — see 'Vessels of the Brain'. 



Modifications of the Envelopes surrounding the Spinal cord. 

 There is little or no splitting of the dura mater, with the consequent 

 formation of an interdural space, within the vertebral column. The 

 only sign of such a condition is towards the anterior end, where the 

 V. post-occipitalis passes through the dura on its way out from the 

 vertebral column. 



On the other hand, there are three well-defined thickenings of the 

 primary vascular coat. 



Two of these, forming a pair of longitudinal bands running one 

 on either side of the spinal cord between the dorsal and ventral nerve 

 roots, were first described by Berger in 1878. He called each the 

 Ugamentum denticulatum. They commence at the level of the second 

 vertebra and continue right down the body, protruding a little dis- 

 tance into the subdural space and are suspended by a reduplication 

 of the vascular coat. 



The third thickening of the primary vascular coat is situated in 

 the mid-ventral line beneath the sulcus longitudinalis ventralis^ and 

 forms the Ugamentum piae ventrale (O'Neill). The spinal cord is 

 somewhat eccentrically placed in the vertebral canal, being rather 

 nearer the ventral side. This brings the ligamentum piae ventrale 

 into very close connexion with the dura mater, and interrupts the 

 subdural space mid-ventrally. 



