IO22 



Ht'.MAN ANATOMY. 







Medulla 



Laminae, cut 



Transverse 



processes 



.. , J - 



Pedicla 



'ural sheath 



XII 1 



Pedicles 





Sheath <>( filum 



Ki 



dura) sheath 



5SH 



tilum 





1 1:1 it 

 t>\ It 



.ral shrath lyini; within 



last 



The Membranes of the 

 Cord. The spinal cord, together 

 with the roots of the thirty-one pairs 

 of spinal nerves, lies within the 

 vertebral canal enclosed by three 

 pn Meeting membranes, ormeninges, 

 which, from without inward, are (i) 

 the dura tnattr, ( 2 ) the aniclinoidca, 

 and (;, i the pia inattr, all of which 

 are directly continuous through the 

 foramen magnum with the corres- 

 ponding coverings of the brain. 

 The external sheath, or thcca, formed 

 by the dura, is a robust fibre-elastic 

 tubular -nvelope, 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 

 plextform veins and loose fatty con- 

 nective tissues ( Fig. ^79). 



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, the_/r7/^w tcr- 

 >ni)uil(\ 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 ,,f an outer fibrous 

 and an inner vascular layer, the 

 connective tissue of the latter ac- 

 companying the blood-vessels into 

 the -nbstance of the cord. 



The arachnoid, a delicate veil- 

 like structure made up of interlacing 

 bundles of libro-elastic tissue, lies 

 between the other two membranes 

 and imots loosely the inner surface 

 of tin- dura and closely the outer 

 surface of the pia. It effectually 

 subdixides the con>idcrable space 

 between the external and internal 

 sheaths into two compartments, the 

 one beneath the dura, the snbditral 

 s/>aff, beini^ little more than a capil- 

 lary cleft tilled with modified lymph, 

 and the other, the subareuJmtrid 

 space, between the arachnoid and 



