318 HUMAN ANATOMY. 



posterior median column from the posterior median fissure. The 

 gray substance occupies the centre of the cord, and is arranged 

 into two crescentic masses connected together by the gray com- 

 missure. The posterior horn forms the apex cornu, from which 

 arises the posterior root of the spinal nerves. The anterior horn 

 is thick and short,, and affords origin to the anterior root of the 

 nerve.' The gray commissure contains throughout its whole 

 length a minute canal the central canal, or ventricle of the 

 cord, continuous above with the fourth ventricle. 



SPINAL NERVES. 



The spinal nerves consist of thirty-one pairs, arranged in 

 the following order : cervical, eight pairs ; dorsal, twelve pairs ; 

 lumbar, five pairs ; sacral, five pairs ; coccygeal, one pair. 



FIG. 146. 



Section of spinal cord and membranes: 1, dura mater; 2, arach- 

 noid membrane; 3, ganglion on posterior root; 4, anterior root of 

 spinal nerve; 5, 5, subarachnoid space; 6, posterior branch of spinal 

 nerve; 7, anterior branch of spinal nerve. 



Each of the spinal nerves arises by two roots, an anterior or 

 motor and a posterior or sensory. The fibres of the anterior 

 root arise from the antero-lateral columns, originating deeply 

 in the gray matter of the cord. The posterior roots arise at the 

 postero-lateral fissure, also originating deeply in the gray matter 

 of the cord. The posterior roots have each a ganglion developed 

 upon it except sometimes the first cervical. These roots unite 

 and the nerve then subdivides into two branches, both having 

 motor and sensory fibres. The posterior branches are smaller 

 and unimportant; they supply the skin and muscles of the back. 



The anterior branches supply the neck, front and sides of 

 the trunk, and the extremities. 



CERVICAL PLEXUS. The cervical plexus is formed by the 

 anterior divisions of the first to the fourth cervical nerves. It 

 is covered by the sterno-mastoid muscle, and rests upon the 



