ANATOMY OF THE NERVOUS SYSTEM. 



161 



B 



D 



F-; 



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 entering the nervous mass 

 beneath. Covering the outside of 

 the pia mater is alayer of flat closely 

 fitting cells; a similar layer lines the 

 inside of the dura mater, and these 

 two layers are described as the third 

 membrane of the cerebro-spinal cen- 

 tre, called the arachnoid. In the 

 space between the two layers of the 

 arachnoid is contained a small quan- 

 tity of watery cerebro-spinal 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 cerebro-spinal 

 liquid. 



The Spinal Cord (Fig. 70) is 

 nearly cylindrical in form, being 

 however a little wider from side to 

 side than dorsoventrally, and taper- 

 ing off at its posterior end. Its 

 average diameter is about 19 milli- 

 meters (f inch) and its length 0.43 

 meter (17 inches). It weighs 42.5 

 grams (1^ ounces). There is no 

 marked limit between the spinal cord 

 and the brain, the one passing grad- 

 ually into the other (Fig. 77)^ but 

 the cord is arbitrarily said to com- 

 mence opposite the outer margin of 

 the foramen magnum of the occipital 

 bone: from there it 'extends to the 

 articulation between the first and 

 second lumbar verteTira^here it 

 narrows off to a slender filament, the different levels. 

 filum terminate (cut off and represented separately at B in Fig 



-8 F 



G 



H 



FIG. 70. The spinal cord and 

 medulla oblongata. A. from the 



