IO24 



HUM AX ANATOMY. 



FIG. 880. 



Skull 

 Vertebral artery 



Spinal 

 accessory nerve 



Pedicles, cut 



Me<lulla 



eessory 



nerve 

 Edge < i 

 cut ilural sheath 



Spinal cord 



Edge ot cut 



dural sheath 





. 



lural 



Filuni rxtonium 

 Hum 



I :...,! 



icrvc :i 



which may transmit blood-vessels, is 

 imperfect or altogether absent. As 

 they cross the suburachnoid space the 

 bundles of root-fibres of the spinal 

 nerves are enclosed by prolongations 

 of the pia and arachnoid. These 

 sheaths are retained by the nerves for 

 only a short distance after the latter 

 receive an additional investment from 

 the dura as they leave the vertebral 

 canal. The dural sheath becomes 

 continuous with the epineurium of the 

 spinal nerves. 



The Cord- Segments. 

 Although no suggestion of such sub- 

 division is to be seen as constrictions 

 on its surface, in principle the spinal 

 cord consists of a series of segments, 

 each of which gives origin to the 

 anterior (motor) and receives \he pos- 

 terior (sensory) root-fibres of one 

 pair of spinal nerves. These nerves, 

 usually thirty-one pairs in number, 

 are classified as eight cervical, twelve 

 thoracic, five lumbar, five sacral, and 

 one coccygeal. Corresponding to the 

 attachment of the nerves the cord is 

 conventionally divided into cervical, 

 thoracic, lumbar, and sacral regions. 

 Of the entire length of a cord measur- 

 ing 43 cm., approximately 10 cm., or 

 about 23. 5 per cent. , belonged to the 

 cervical region; 24 cm., or 55.5 per 

 cent., to the thoracic; 6 cm., or 14 

 per cent., to the lumbar; and 3 cm., 

 or 7 per cent., to the sacral region. 

 The spinal nerves are attached 

 to tin- lateral surfaces of the cord by 

 fan-shaped groups of anterior and pos- 

 terior ro..t-til. res that are gathered into 

 compact strands as they converge to 

 form a common trunk (Fig. 884). 

 The portion of the spinal cord with 

 which the ro, ,t -fibres of a spinal nerve 

 an- connected constitutes its cord- 

 segment, the limits of which lie in the 

 interval separating the extreme fibres 

 of the nerve and those of the adjacent 

 nerves. In the thoracic cord these 

 intervals are very evident, since the 

 .inents are relatively long; in the 

 cer\ical and lumbar regions, on the 

 contrary, the groups of root-fibres 

 are so crowded that they form almost 

 unbroken rows, 



The length of the individual cord- 

 varies; thus, according to the 

 of Liideritz, those of the 

 region, are from 11-13.5 mm.; 



