SPINAL NERVES. 377 



lower pointed extremity of the cord. Internally it is united to the pia 

 mater. Externally it ends in a series of triangular or tooth-like projec- 

 tions (of), which are fixed at intervals into the dura mater, each being 

 about midway between the apertures of the roots of the spinal nerves. 

 There are twenty or twenty-one denticulations ; of which the first is at- 

 tached to the dura mater opposite the margin of the occipital foramen, and 

 the last, opposite the twelfth dorsal, or the first lumbar vertebra. 



This fibrous band supports the spinal cord, and has been called a liga- 

 ment from that circumstance. 



Vessels and nerves of the membranes. The dura mater of the cord has 

 but few vessels in comparison with that in the skull, for its office is differ- 

 ent. Nerves are said to be furnished to it from offsets on the vessels sup- 

 plying the cord. 



The arachnoid is sparingly supplied with vessels like serous membranes 

 in general ; and proof of its containing nerves, in man, is yet wanting. 



The pia mater has a network of vessels in its substance, though this is 

 less marked than in the part on the brain ; and from them offsets enter 

 the cord. In the membrane are many nerves, derived from the sympa- 

 thetic. 



Dissection. The arachnoid membrane is to be taken from the fibrils 

 of the roots of the nerves on one side ; and the roots are to be traced out- 

 wards to their transmission through two apertures in the dura mater for 

 each nerve. 



One of the offsets of the dura mater, which has been cut of some 

 length, is to be laid open to show the contained ganglion. The student 

 should define one ganglion, showing its bifid condition at the inner end 

 (fig. 123, B ), and should trace a bundle of threads of the posterior root into 

 each point. The anterior root, consisting of two bundles of threads, is 

 to be followed over the ganglion to its union with the posterior beyond the 

 ganglion. 



SPINAL NERVES. There are thirty-one pairs of spinal nerves ; and 

 each nerve is constructed by the blending of two roots (anterior and pos- 

 terior) in the intervertebral foramen (fig. 123). 



They are divided into groups corresponding with the regional subdi- 

 visions of the spinal column, viz., cervical, dorsal, lumbar, sacral, and 

 coccygeal. In each group the nerves are the same in number as the ver- 

 tebne, except in the cervical region of the spine where they are eight, and 

 in the coccygeal region where there is only one. In consequence of the 

 number of nerves in the neck, exceeding that of the vertebrae, the last is 

 placed below the seventh vertebra ; and the lowest nerve of each group, 

 except the coccygeal, will be below its corresponding vertebra. 



Each nerve divides into two primary branches, anterior and posterior, 

 the former of these is distributed to the front of the body and the limbs, 

 and the latter is confined to the hinder part of the trunk. 



ROOTS OP THE NERVES (fig. 123). Two roots (anterior and posterior) 

 attach the nerve to the spinal cord ; and these unite together to form a 

 common trunk in the intervertebral foramen. The posterior root is 

 marked by a ganglion, but the anterior root is aganglionic. 



The posterior or (/anglionic roots (fig. 123, b) surpass in size the an- 

 terior, and are formed by larger and more numerous fibrils. They are 

 attached to the side of the cord between the posterior and lateral columns 

 in a straight line, which they keep even to the last nerve. 



In their course to the trunk of the nerve the fibrils converge to an aper- 



