ANATOMY OF THE NERVOUS SYSTEM 



121 



D 



E 



with the proper nervous parts, is the pia mater, also made up 



of white fibrous tissue interwoven with A 



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 be- 



fore entering the nervous mass beneath. 



Covering the outside of the pia mater is 



a layer of flat closely fitting cells; a simi- 



lar layer lines the inside of the dura 



mater, and these two layers are described 



as the third membrane of the cerebro- 



spinal center, called the arachnoid. In 



the space between the two layers of the 



arachnoid is contained a small quantity 



of watery cerebrospinal liquid. The sur- 



face 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 contained some of the 



cerebrospinal liquid. 



The Spinal Cord (Fig. 58) is nearly 

 cylindrical in form, being however a lit- 

 tle wider from side to side than dorsiven- 

 trally, and tapering off at its posterior 

 end. Its average diameter is about 19 

 millimeters (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 gradually into the other 

 (Fig. 64) , but the cord is arbitrarily said 

 to commence opposite the outer margin 

 of the foramen magnum of the occipital 

 bone: from there it extends to the articu- 



F 



H 



3! 

 the ventral, and B, from 



the dorsal aspect ; C to H cross- 



lation between the first and second lum- sections at different levels. 

 bar vertebra, where it narrows off to a slender non-nervous fila- 



