296 



STRUCTURE OF THE SPINAL CORD. 



terior furrows : 6, white or anterior commissure ; 7, gray or posterior 

 commissure ; 8, anterior horn of right crescent ; 9, posterior horn of dit- 

 to : it is prolonged to the posterior furrow ; 10, antero-lateral columns ; 

 11, 11, posterior columns: these are all of white tubular substance. The 

 symmetrical reference numbers on one side are omitted for the sake of 

 clearness. 



The spinal cord is surrounded by three membranes, continuous with 

 Membranes of those of the cranium : the dura mater, the arachnoid, and the 

 the spinal cord. pi a mater. The latter embraces the cord so closely as to ex- 

 ert a compression upon it. This is shown on slightly wounding it,' when 

 the white substance protrudes through the orifice. Fig. 140. 



Fig. 140 : 1, spinal dura mater laid open and drawn 

 aside ; 2, 2, sheaths formed by this membrane round 

 the roots and spinal ganglia; 3, spinal arachnoid; 

 4, 4, sheaths formed by the arachnoid around the 

 roots of the nerves and dentated ligament ; 5, 5, points 

 of communication of the visceral layer -of the arach- 

 noid, with its parietal layer ; 6, pia mater ; 7, denta- 

 ted ligament separating the anterior roots from the 

 posterior roots of the spinal nerves, and serving as a 

 communication between the dura mater and pia mater. 



From the spinal cord there arise thirty-one pairs 

 The spinal f nerves, each nerve having two roots, an 

 nerves. anterior or motor, and a posterior or sensory. 



The anterior roots issue from the anterior furrow, 

 Roots of the the posterior from the posterior furrow, 

 spinal nerves. w here the gray substance emerges. Of 

 the two the latter are the larger, and have more radicles. They also 

 have, in the intervertebral foramen, a ganglion. Beyond the ganglion the 

 two roots coalesce, and the resulting nerve trunk, passing through the 

 intervertebral foramen, divides into an anterior and posterior branch, for 

 the anterior and posterior portions of the body. To this general descrip- 

 tion there are, however, some exceptions. Thus the posterior root of the 

 first cervical nerve is smaller than the anterior, and very often it has no 

 ganglion. The spinal nerves are enumerated as eight cervical, twelve 

 dorsal, five lumbar, and six sacral pairs. The cervical pass off to their 

 distribution transversely, the dorsal obliquely, and the lumbar and sacral 

 vertically. The latter constitute the cauda equina. 



Fig. 141 illustrates the origin of the anterior roots of the spinal 

 nerves. 1, pons varolii; 2, large and small root of the fifth pair; 3, 

 sixth pair; 4, facial nerve; 5, auditory nerve; 6, intermedian nerve; 

 7, glosso-pharyngeal ; 8, pneumogastric ; 9, spinal accessory ; 10, hypo- 

 glossal. 



Spinal dura mater laid 

 open. 



