276 



THE SPINAL NERVES. 



The ganglia are in general placed in the intervertebral foramina, immediately 

 beyond the spots at which the roots perforate the dura mater lining the spinal canal. 

 The first and second cervical nerves, however, which do not pass through inter- 

 vertebral foramina, have their ganglia in the corresponding position as they lie over 

 the neural arches of the vertebras. The ganglia of the sacral nerves are contained 

 in the spinal canal, that of the last nerve being occasionally at some distance from 

 the point at which the nerve issues. The ganglion of the coccygeal nerve is placed 

 in the canal within the sac of the dura mater, and at a variable distance from the 

 origin of the nerve. 



The filaments of the posterior root of the nerve are collected into two bundles as 

 they approach the ganglion, and the inner extremity of the oval-shaped ganglion is 

 usually bilobate, the lobes corresponding to the two bundles of filaments. 



The anterior roots of the spinal nerves are smaller than the posterior, and are 

 devoid of ganglia, their fibres arising from the cells of the grey matter of the spinal 



Fig. 186. DIFFERENT VIEWS OF A POR- 

 TION OF THE SPINAL COUI) FROM THE 

 CERVICAL REGION WITH THE ROOTS 



OF THE NERVES. Slightly enlarged. 

 (Allen Thomson. ) 



In A, the anterior surface of the speci- 

 men is shown, the anterior nerve-root of 

 the right side being divided ; in B, a 

 view of the right side is given ; in C, the 

 upper surface is shown ; in D, the nerve- 

 roots and ganglion are shown from below : 

 1, anterior median fissure ; 2, posterior 

 median furrow and septum ; 3, origin 

 of the anterior nerve- roots ; 4, postero- 

 lateral groove, into which the filaments 

 of the posterior root are seen to sink ; 5, 

 anterior root passing over the ganglion ; 

 5', in A, the anterior root divided ; 6, 

 posterior root, the fibres of which enter 

 6', the ganglion ; 7, the antei-ior, and 

 7', the posterior primary division of the 

 nerve, each of which is seen in A and D 

 to be derived in part from the anterior 

 and in part from the posterior root. 



cord, and mainly from those of the anterior cornu. Their filaments also tend to be 

 collected into two bundles near the intervertebral foramen. 



Both anterior and posterior roots of all the spinal nerves contain fibres of very various 

 sizes large, medium (10 /ti to 5 /z), and fine, but the proportion in which the different groups 

 occur is not the same throughout. The anterior roots of the cervical, lumbar, and sacral 

 nerves consist mainly of large and medium-sized fibres, fine fibres being present only in small 

 number, generally not exceeding- one-sixth of the whole. Those of the dorsal nerves, however, 

 as well as the coccygeal. show a preponderance of fine fibres, which are about three times as 

 numerous as the larger ones. The finest fibres (2'6 p. and less) are abundant in the anterior 

 roots of the dorsal nerves, but rare in the other regions, except in the last two sacral and the 

 coccygeal nerves. In the posterior roots the small fibres are about equal in number to the 

 medium-sized and large fibres together ; and. fibres of the largest size (above 20 /u). which are 

 frequent in the anterior roots, are here scanty. (E. Siemerling, Neurol. Centralbl., 1886, and 

 ' Anatomische Untersuchungen iiber die menschlichen Ruckenniarkswurzeln," Berlin, 1887.) 



Varieties. The posterior root of the first cervical nerve is sometimes wanting (8 percent., 

 Kazzander) ; or it may arise partly or wholly from, or in common with, the spinal accessory 

 nerve (see p. 270). A defect in the roots of the thoracic nerves is said to be very common by 

 Adamkiewicz. who found in sixteen spinal cords only three with the full number of thoracic 

 roots ; in three there was absence of both anterior and posterior roots of one nerve, in three 

 absence of a posterior root only, and in seven absence of one anterior root (Virchow's Archiv. 

 Ixxxviii, 1882). Communications between the root-filaments (especially the posterior) of 

 adjoining nerves are frequently met with. Small detached portions of the intervertebral 



