34 BIOLOGICAL PHYSICS 



by it, until the accomplishment of the neurenteric differen- 

 tiation and the subsequent development of the rudimentary 

 cranio-spinal canal, with which it is at first conterminous, 

 the succeeding stages being characterised by a gradual 

 shortening of the canal, as its walls undergo a thickening 

 as the neural structures increase, during which it recedes 

 from the tip of the rudimentary coccyx to the first or 

 second lumbar vertebra, where it afterwards remains, leav- 

 ing behind it a thecal continuation, known as the filum 

 terminale, to maintain a modified structural and functional 

 connection with the posterior orifice of the enteric canal, 

 with which it was formerly continuous, to provide a, 

 posterior exit for this neural lymph or fluid. Great but 

 decreasing patency continues to characterise the spaces con- 

 taining the cerebro-spinal fluid, and the utmost regulated 

 freedom is provided for its excretion at the various points 

 of exit, until, by increasing age, consequent stiffening of 

 textures and blockage of spaces, as well as exits, a condi- 

 tion of comparative stasis ensues as life advances to its 

 close, and death follows. 



