THE DISSECTION OF THE BACK 189 



In length it measures about six inches. The part within 

 the tube of dura is termed the filum terminale internum, the 

 portion outside is \hefilum terminale externum. 



Spinal Nerves. Thirty-one spinal nerves take origin from 

 each side of the medulla spinalis (O.T. spinal cord). These 

 are classified into five groups, according to the vertebrae with 

 which they are associated. The thoracic, lumbar, and sacral 

 nerves correspond in number with the vertebras in each of 

 those regions thus, there are twelve thoracic, five lumbar, 

 and five sacral nerves, each of which issues from the vertebral 

 canal below the vertebra with which it numerically corresponds. 

 In the cervical region, however, there are eight nerves, the 

 first of which comes out between the occiput and the atlas, 

 and is therefore distinguished by the special name of the sub- 

 occipital nerve. There is only one coccygeal nerve on each 

 side. 



Spinal Nerve-Roots (Figs. 75 and 77). Each spinal nerve 

 springs from the side of the spinal medulla by two roots an 

 anterior 'and a posterior. Except in the case of the sub-occipital 

 nerve (where the posterior root is sometimes absent), the 

 posterior nerve-root is the larger of the two. In addition, 

 the posterior root is distinguished by possessing an oval 

 ganglion, termed the spinal ganglion. There is, also, a 

 wide physiological difference between the two roots the 

 posterior root is composed of afferent fibres, the anterior root 

 consists of efferent fibres. Immediately beyond the ganglion 

 the two roots unite to form the spinal nerve-trunk, which 

 contains a mixture of both efferent and afferent nerve-fibres. 



The mode of attachment of the two nerve-roots to the side of 

 the medulla spinalis is somewhat different in the two cases. 

 In each instance they are attached by several separate fila 

 radicularia, which spread out from each other as they approach 

 their attachments. In the case of the posterior root the 

 fila enter the spinal medulla consecutively along a continuous 

 straight line and at the bottom of a slight furrow. The fila 

 of the anterior root, on the other hand, are not so regularly 

 placed. They emerge from the medulla spinalis over an area 

 of some breadth. The portion of the medulla spinalis which 

 stands in connection with a pair of nerves receives the 

 name of a "neural segment." 



It will be noted that the size of the nerve-roots differs greatly. 

 The lower lumbar and upper sacral nerve-roots are much 



