/'///-; SPINAL \Kitv i-:x 



fibres. Thus the \\hite ranii have been tcnnc;! the visceral divisions of the spinal 

 nerves. The grey raini consist chiefly of sympathetic 111. res, most of which are non- 

 medullafed or partially medullated, and which COUT86 to their (list ribut ion by way of 

 the spinal nerves. Some of the sympathetic lilires terminate in the spinal ganglion, 

 afferent sympathetic fibres (fig. <>7:5). The usual absence of white rami communi- 

 cantes from the cervi.-al nerves is explained on the grounds (1) that probal.lv 

 relatively few efferent splanchnic fibr, to the sympathetic from this 



region of the cord: ii>) tluit many of the ellerent splanchnic fibres which do arise 

 from this region of the cord probably join the rootlets of the spinal accessory or 

 eleventh cranial nerve and pa-s to the sympathetic system through the trunk of 'this 

 nerve, and through the vagus with which it anastomose-; and (3) that such of these 

 fibres as are given off from the lower segments of this region, descend the cord and 

 pass out by way of the upper thoracic nerves which give very evident white rami to 

 i he sympathetic. 



The meningeal or recurrent branch (figs. 672, 673, and 685) is very small and 

 variable, and is seldom seen in ordinary dissections. It is given off from the nerve- 

 trunk just before its anterior and posterior primary divisions are formed. It con- 

 sists of a few peripheral branches of spinal ganglion-cells (sensory fibres) which leave 

 the nerve-trunk and re-enter the vertebral canal for the innervation of the meninges, 

 and which are joined by a twig from the grey ramus or directly from the nea> 

 sympathetic ganglion (vaso-motor fibres). There is considerable evidence, both 

 physiological and anatomical, obtained chiefly from the animals, which shows that 

 at times certain of the peripheral spinal ganglion or sensory fibres may turn back- 

 wards in the nerve-trunk and pass to the meninges within the ventral root instead 

 of contributing to a recurrent branch. The occurrence of such fibres in the ventral 

 root explains the physiological phenomenon known as 'recurrent sensibility.' Like- 

 wise, sympathetic fibres entering the trunk through the grey ramus may pass to the 

 meninges by way of the ventral root, and at times the recurrent branch is probably 

 absent altogether, its place being taken entirely by the meningeal fibres passing in 

 the ventral root. 



Areas of distribution of the spinal nerves. Both the posterior and anterior 

 primary divisions divide and subdivide repeatedly, and their component fibres are 

 distributed to areas of the body more or less constant for the nerves of each pair, but 

 the distribution of the different nerves is very variable. Corresponding to their 

 attachment, each to a given segment of the spinal cord, the nerves have primarily 

 a se<.:mental distribution, but, owing to the developmental changes and displacement 

 of parts during the growth of the body, the segmental distribution becomes greatly 

 obscured and in some nerves practically obliterated. Naturally it is more retained 

 by the nerves supplying the trunk than by those contributing to the innervation of 

 the limbs and head, and the areas supplied by the posterior primary divisions are 

 less disturbed than those supplied by the anterior. The segmental areas of cutaneous 

 distribution of the posterior divisions are more evident than the areas of muscle 

 supplied by these divisions, from the fact that the segmental myotomes from which 

 the dorsal muscles arise fuse together and overlap each other "considerably during 

 development. No nerve has a definitely prescribed area of distribution, cutaneous 

 or muscular, for its area is always considerably overlapped by the areas of the nerves 

 adjacent. to it. The mid-thoracic nerves more nearly supply a definitely prescribed 

 belt of the body. 



POSTKRIOH 1'KIMARY DIVISIONS 



The posterior primary divisions of the spinal nerves spring from the trunks 

 immediately outside the intervertebral foramina, and they pass backwards between 

 the adjacent transverse processes. With I he except ions df the first and second cer- 

 vical nerves they are smaller than the corresponding anterior primary divisions, and 

 after passing between the transverse processes into the region of the back thev 

 divide into medial and lateral branches. This division, however, does not occur 

 in the cases of the first cervical, the last two sacral, and the coccygeal nerves. 



CKRVICM. XKKVKS 



The posterior primary division of the first cervical or sub-occipital nerve 



is larger than the anterior primary division. It springs from the trunk, between 



