A STUDY OF THE NERVOUS SYSTEM 



257 





Ventral nerve-root 



and ventral fissures the spinal cord is partly divided into 

 right and left halves (see Fig. 117). 



Coverings of the Cord. Three distinct coverings surround 

 the cord. The outside one is a loose sheath of tough con- 

 nective tissue ; it is called the dura mater (Latin dura = 

 hard + mater = mother, probably because of the protec- 

 tion it affords). 

 The innermost of 

 the three mem- 

 branes, the pia 

 mater (Latin pia 

 = gentle + mater 

 = mother), is a 

 thin covering, 

 well supplied with 

 blood vessels, and 

 is closely at- 

 tached to the cord. 

 Between the dura 

 and pia mater is 

 a third layer of 

 loose connective 

 tissue with an 

 appearance some- p a , urtorp , lin . ry- 

 thing like that Anterior^ 

 of a spider's web ; 



from this fact it is FlG ' 116> ~~ Piece of s P inal Cord, showing its Three 



Coverings and the Roots of the Spinal Nerves, 

 called the a-rach 1 - 



noid (Greek, meaning like a spider's web). Within the 

 meshes of the arachnoid is a watery fluid (cerebro-spinal 

 fluid) somewhat like lymph in composition. The spinal 

 cord, then, is successively wrapped about and protected by 

 the following coverings, skin, muscle, bony arches of the 

 vertebrae, dura mater, arachnoid with its liquid, and pia 

 mater. 



Cross Section of the Cord. When one looks at the cross 



Dorsal nerve-root 



Dorsal ganglion 



'ntrl nerve-raot 



Spinal cord 



