1312 A MANUAL OF ANATOMY 



Internally it forms an uninterrupted band which is attached to 

 the pia mater along the lateral column of the cord. Externally 

 it is denticulated. The denticulations (about twenty-one in number) 

 carry with them sheaths from the arachnoid, and their pointed 

 outer ends are attached to the inner surface of the theca in the 

 intervals between each pair of openings for the exit of the ventral 

 and .dorsal nerve-roots. 



The two ligamenta denticulata act as lateral supports to the spinal 

 cord. 



Structure. — Each ligamentum denticulatum consists of fibrous tissue, 

 which is continuous with the outer layer of the pia mater. 



External Characters of the Spinal Cord. — ^The spinal cord, or 

 medulla spinalis, which is somewhat cylindrical, is that division of 

 the cerebro-spinal axis which is situated within the spinal canal. 

 It extends from the loWer margin of the foramen magnum in the 

 occipital bone to about the level of the disc between the bodies ot 

 the first and second lumbar vertebrae, and it is about i8 inches 

 in length. Superiorly it is continuous with the medulla oblongata, 

 and interiorly it terminates in a tapering portion, called the conus 

 meduUaris. From the lower end of this cone a slender,- glistening 

 thread, called the fllum terminale, is continued downwards between 

 the bundles of lumbar, sacral, and coccygeal nerves of either side, 

 which constitute the cauda equina, to be attached to the back of 

 the first coccygeal segment. 



The spinal cord is of smaller dimensions than the spinal canal, 

 and is therefore relieved from pressure during the ordinary move- 

 ments of the vertebral column. It is surrounded by three 

 membranes, already described, namely, the dura mater, the 

 arachnoid membrane, and the pia mater, in this order from with- 

 out inwards. Within its theca of dura mater the cord is sus- 

 pended by means of the ligamenta denticulata, and the nerve- 

 roots as they emerge through the openings in the theca. 



The spinal cord varies in shape in different regions. In the 

 cervical region, as seen in transverse section, it is transversely 

 oval, and is slightly flattened from before backwards. In the 

 thoracic region it is somewhat circular, but the transverse 

 diameter exceeds the antero-posterior. In the lumbar region it is 

 still more circular than in the thoracic region. 



The cord presents two swellings, which are known as the cervical 

 and lumbar enlargements, and which are associated with the 

 numerous and large nerve-trunks- destined for the upper and lower 

 limbs. The cervical enlargement extends from near the com- 

 mencement of the cord to the second thoracic vertebra, and its 

 breadth is greatest opposite the sixth cervical vertebra. The lumbar 

 enlargement, which is less conspicuous than the cervical, extends 

 from the level of the tenth thoracic vertebra to the conus medul- 

 laris, and its breadth is greatest opposite the twelfth thoracic 

 vertebra. 



