I022 



HUMAN ANATOMY. 



Fig. 877. 



Skull 



Pedicles, cut • 



8 en. 



Pedicles. 



XII T- 



12 tn - 



Pedicles< 



f. 



:ln. 



1 sn — 



Medulla 



Posterior 



divisions of 

 sacral nerves < 



Sheath of filiim  



'«( 



^-y- 



• Laminae, cut 



-Transverse 



processes 



^■iit^ 



l>f'-..: 



-D'.iral sheath 



t 



}^ 



;>^ 



'-4J 



l^v. 



-7 In 



i/,-l 



.VL 

 -5 In 



'-A 



End of 



dural sheath 



5 sv 



End of filum 



, Coccvx 



Spinal cord enclosed in nnopetied dural sheath lyiiig within 

 vertebral canal ; neural arches completely removed on right side, 

 partially on left, to expose dorsal aspect of dura; first and last 

 nerves of cervical, thoracic, lumbar and sacral groups ar,e indicated 

 by Italic figures; corresponding vertebra; by Roman numerals. 



The Membranes of the 

 Cord. — The spinal cord, together 

 with the roots of the thirty-one pairs 

 of spinal nerves, lies -.vithin the 

 vertebral canal enclosed by three 

 protecting membranes, or meninges, 

 which, from without inward, are (i) 

 the dura mater, (2) the arac/moidea, 

 and (3) the/m mater, all of which 

 are directly continuous through the 

 foramen magnum with the corres- 

 ponding coverings of the brain. 

 The external sheath, or theca, formed 

 by the dura, is a robust fibro-elastic 

 tubular envelope, much longer and 

 considerably wider than the cord, 

 that does not lie against the wall of 

 the vertebral canal, but is separated 

 by an interval containing thin-walled 

 plexiform veins and loose fatty con- 

 nective tissues (Fig. 879). 



The dural sheath, about .5 

 mm. in thickness, extends to the 

 level of the second sacral vertebra 

 and is, therefore, considerably longer 

 than the spinal cord. The part of 

 the sac not occupied by the cord 

 encloses the longitudinal bundles 

 of root-fibres, that pass obliquely to 

 the levels at which the correspond- 

 ing nerves leave the vertebral canal, 

 and a fibrous strand, \\\^ filum ter- 

 minale, prolonged from the cord to 

 the lower end of the spine. 



The pia constitutes the imme- 

 diate investment of the cord and 

 supports the blood-vessels destined 

 for the nutrition of the enclosed 

 nervous cylinder. The pial sheath 

 is composed of an outer fibrous 

 and an inner vascular layer, the 

 connective tissue of the latter ac- 

 companying the blood-vessels into 

 the substance of the cord. 



The arachnoid, a delicate veil- 

 like structure made up of interlacing 

 bundles of fibro-elastic tissue, lies 

 between the other two membranes 

 and invests loosely the inner surface 

 of the dura and closely the outer 

 surface of the pia. It effectually 

 subdivides the considerable e^pace 

 between the external and internal 

 sheaths into two compartments, the 

 one beneath the dura, the subdural 

 space, being little more than a capil- 

 lary cleft filled with modified lymph, 

 and the other, the subarachnoid 

 space, between the arachnoid and 



