542 



THE SPINAL CORD AND ITS MEMBRANES. 



attachment 

 of points. 



Vessels and 

 nerves of 

 dura mater 



of arach- 

 noid; 



Dissection 

 of roots of 

 nerves, 



and the 

 ganglion. 



and ends below on the pointed extremity of the cord, Internally it 

 is united to the pia mater. Externally it ends in a series of tri- 

 and on other angular or tooth-like projections (d), which are fixed at intervals 

 mater 1 - 1 ^ nto tne ^ ura mater > eac ^ being about midway between the aper- 

 number and tures of the roots of the spinal nerves. There are twenty or twenty- 

 one denticulations, of which the first is attached to the dura mater 

 opposite the margin of the occipital foramen, and the last, opposite 

 the twelfth dorsal or the first lumbar vertebra. 



Vessels and nerves of the membrane. The spinal dura mater has 

 but few vessels in comparison with that in the skull, as it has 

 not the same periosteal office. Filaments of the sympathetic and 

 spinal nerves are furnished to the membrane. 



The arachnoid has no vessels ; and proof of its containing nerves 

 in man is yet wanting, 

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

 this is less marked here than on the brain ; and from them offsets 

 enter the cord. In the membrane are many nerves derived from 

 the sympathetic and the posterior roots of the spinal nerves. 



Dissection. The arachnoid membrane is to be taken away on 

 one side ; and the nerve roots are to be traced outwards to their 

 transmission through apertures in the dura mater. 



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

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

 student should define the ganglion, showing its bifid condition 

 at the inner end (fig. 196, 6), and should trace a bundle of 

 threads of the posterior root into each point. The anterior root, 

 consisting also of two bundles of threads, is to be followed over 

 the ganglion to its union with the posterior root beyond the 

 ganglion. 



SPIN A.L NERVES. There are thirty-one pairs of spinal nerves; 

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

 and posterior) in the intervetebral foramen. 



They are divided into groups corresponding with the regional 

 and groups, subdivisions of the spinal column, viz., cervical, dorsal, lumbar, 

 sacral, and coccygeal. In each group the nerves are the same in 

 number as the vertebras, except in the cervical region, where 

 they are eight, and in the coccygeal region where there is only one. 

 Relation of The cervical nerves from the first to the seventh pass out above the 

 vStebr. several vertebrae ; and the eighth is below the last cervical vertebra ; 

 the succeeding nerves are placed each 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, ROOTS OF THE NERVES (fig. 196). Two roots (anterior and 



poster?or and posterior) attach the nerve to the spinal cord ; and these unite 



together to form a common trunk in the inter vertebral foramen. 



The posterior root is marked by a ganglion, but the anterior root is 



aganglionic. 



Trunks of 



spinal 



nerves. 



Number 



Primary 

 divisions. 



