ANATOMY OF THE NERVOUS SYSTEM 



141 



D 1' 



This is the pia mater, also made up of A 

 white fibrous tissue interwoven with 

 elastic fibers, but less closely than in 

 the dura mater, so as to form a less dense 

 and tough membrane. The pia mater 

 contains many blood-vessels which break 

 up in it into small branches before enter- 

 ing the nervous mass beneath. Covering 

 the outside of the pia mater is a layer of 

 flat closely fitting cells; a similar layer 

 lines the inside of the dura mater, and 

 these two layers make up the third mem- 

 brane, called the arachnoid. In the space 

 between the two layers of the arachnoid 

 is contained a small quantity of watery 

 cerebrospinal liquid. The surface of the 

 brain is folded and the pia mater follows 

 closely these folds; the arachnoid often 

 stretches across them: in the spaces thus 

 left between it and the pia mater is con- 

 tained some of the cerebrospinal liquid. 

 Ventricles of the Brain and Central 

 Canal of the Spinal Cord. The central 

 nervous system begins its embryonic 

 development as a groove in the layer of 

 cells which forms the upper surface of the 

 embryo. This groove deepens, and finally 

 cuts itself off from the cell layer of which 

 it was at first a part. The edges grow to- 

 gether transforming the groove into a 

 tube. The cell layer heals over, leaving 

 the neural tube beneath it. The hollow in 

 the tube persists throughout life. In the 

 adult spinal cord it is represented by the 

 tiny central canal (Fig. 65). As the front 

 end of the neural tube develops into the gom the ventral, and B, from 



^ the dorsal aspect ;C to H, cross- 



COmplex brain the hollow in this region sections at different levels. 



takes the form of a series of chambers of extremely irregular shape, 

 communicating with each other and with the central canal of the 



G 



H 



C A, 



