SPINAL CORD AND MEMBRANES. 135 



length, and separating the anterior and posterior roots of 

 the spinal nerves, is a white fibrous band, connected inter- 

 nally with the pia mater, and having about twenty serra- 

 tions along its free margin which connect it externally with 

 the dura mater; from this peculiarity, and from its sup- 

 porting the cord, it receives the name of ligamentum denti- 

 culatum, or membrana dent at a. 



The SPINAL CORD gives off thirty-one pairs of nerves, 

 arising by two roots and passing out at the intervertebral 

 foramina ; they are divided into groups, which are named 

 cervical (eight pairs), dorsal (twelve pairs), lumbar (five 

 pairs), sacral (five pairs), and coccygeal (one pair) ; in. 

 each group the nerves are equal to the number of vertebrae, 

 except in the cervical, which has eight, and in the coccy- 

 geal, which has but one ; as the cervical nerves exceed the 

 number of cervical vertebrae, the lowest nerve of each 

 group is consequently below its corresponding vertebra. 



The two roots which blend to form the spinal nerves are 

 called anterior and posterior, or ganglion ic and a-ganglio- 

 nic. The posterior roots are the largest, and are each fur- 

 nished with a ganglion. As the apertures for the trans- 

 mission of the nerves are not opposite their points of 

 origin, they get an oblique direction, increasing from above 

 downward ; in the lumbar and sacral region their direction 

 is vertical, and the collection of the roots of the nerves 

 around the filum terminale, which constitutes about one- 

 third of the whole length of the cord, is called the cauda 

 equina. It is upon these lower nerves that the ganglion of 

 the posterior root may be best^observed, as in those nerves 

 given off more nearly opposite their foramina it often lies 

 in the intervertebral canal. The first nerve sometimes 

 wants a posterior root. 



In the upper part of the canal, the spinal portion of the 

 spinal accessory nerve (p. 83) should be sought ; it arises 

 by fine filaments from the side of the spinal cord as low 

 down as the sixth cervical nerve, and lies between the mem- 

 brana dentata and the posterior roots of the spinal nerves, 

 with the upper of which it is sometimes connected; it 

 finally enters the skull by the foramen magnum, to join the 

 accessory portion. 



The spinal cord is supplied by several offsets from the ver- 

 tebral arteries; near their termination in the basilar artery 

 they give off two branches which unite under the name of 

 the anterior spinal artery; this is continued to the bottom of 



