THE VERTEBRAL COLUMN. 



485 



the dorsal spinal veins. The dura 

 mater is almost never torn in in- 

 juries even though the cord may be 

 crushed (Fig. 487). 



Arachnoid. The arachnoid 

 of the spinal cord is a stouter mem- 

 brane than the cerebral arachnoid. 

 Above it is continuous with the cer- 

 ebral arachnoid at the foramen mag- 

 num. Below it blends with the dura 

 at about the level of the third sacral 

 vertebra. Thus it is seen that while 

 the cord itself ends at the lower bor- 

 der of the first lumbar vertebra the 

 subarachnoid cavity is prolonged 

 nearly or quite to the third sacral. 

 As in the brain, the cavity between 

 the arachnoid and the dura above is 

 slight, the two membranes being 

 practically in contact, so that there 

 is almost no subdural space. Be- 

 tween the arachnoid and pia, how- 

 ever, there is a considerable cavity 

 which is continuous with the same 

 space beneath the cerebral arachnoid. 

 It communicates with the fourth 

 ventricle just above the calamus 

 scriptorius by the foramen of Ma- 

 gendie in the median line, and at the 

 sides by the foramina of Key and 

 Retzius, and also by slits at the de- 

 scending horns of the lateral ventri- 

 cles. Hence it is that the ventricular 

 fluid can be drained by a lumbar 

 puncture. 



Through this subarachnoid 

 space pass the septum posticum be- 

 hind and the ligamenta denticulata 

 on each side from the pia to the 

 dura mater. It is also traversed by 

 the anterior and posterior roots of 

 the spinal nerves, the former being in 

 front and the latter behind the liga- 

 mentum denticulatum. The arach- 

 noid contains neither vessels nor 

 nerves (Fig. 488). 



Pia Mater. The spinal pia 

 mater is thin and closely invests the 

 cord. It carries /the blood-vessels 

 of the cord and sends prolongations 

 posteriorly to the dura as the septum 

 posticum, laterally as the two liga- 

 menta denticulata, and also around 

 the anterior and posterior roots of 

 the spinal nerves. 



Spinal Vessels. Three ar- 

 teries supply the spinal cord, an ante- 

 rior spinal in the median line of the 

 anterior surface and two posterior 



Skull 



Pedicles, cut 



Pedicles< 



XII T- 



12 tt: 



Pedicles< 



Laminae, cut 



Transverse 



processes 



Dural sheath 



wd XII T 

 12 in 



.1 In 



End of 



dural sheath 



Posterior 

 divisions of 

 sacral nerves * 



Sheath of filum 



End of filum 



, Coccyx 



FIG. 487. Spinal cord enclosed in unopened dural sheath 

 lying 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 are indicated by Italic figures; corresponding vertebrae 

 by Roman numerals. (Piersol.) 



