ANATOMY OF THE NERVOUS SYSTE\f 



121 



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

 of 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 be- 

 fore entering 1 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 r on * the ventral, ami 



...... the dorsal aspect ;C to a cross- 



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

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



