526 BIOLOGICAL PHYSICS 



act as so many percolators, through which an over- 

 compressed cerebro-spinal fluid can find its way out of 

 the cranial cavity into the porous substance of the 

 superimposed bones. In the closely clinging pia mater 

 the blood-vessels are conveyed which nourish and sustain 

 the cerebral and spinal nervous structures. Thus is pro- 

 vided a series of structures of most beautiful texture, and 

 most elaborate design and detail, to ensure the safety and 

 nutrition of the most wonderful living mechanism known 

 to science. 



