970 THE NERVOUS SYSTEM 



ramus). Where there are two, one usually contains medullated fibres, chiefly 

 from the ventral root, sufficient to give it a whiter appearance (white ramus) . 



In the upper cervical and in the sacral regions one sympathetic gangUon may be connected 

 with two or more spinal nerves, and sometimes one nerve is connected with two ganglia. The 

 rami communicantes of the spinal nerves are equivalent to the communicating rami connecting 

 certain of the cranial nerves with the sympathetic system (trigeminus, glosso-pharyngeus, vagus). 

 The medullated fibres of the rami and, therefore, the white rami consist chiefly of fibres from the 

 spinal nerves, viz., fibres from the spinal ganghon-cells which enter and course to their distri- 

 bution through branches of the sympathetic nerves, visceral afferent fibres, and fibres from 

 the ventral roots of the spinal nerves which terminate in the sympathetic ganglia, visceral 

 efferent (preganghonic) fibres. Thus the white rami have been termed the visceral divisions 

 of the spinal nerves. The grey rami consist chiefly of sympathetic fibres, most of which are 

 non-medullated or partially medullated, and which course to their distribution by way of 

 the spinal nerves. Some of the sympathetic fibres terminate in the spinal gangUon, afferent 

 sympathetic fibres (fig. 748). The usual absence of white rami communicantes from the cervical 

 nerves is explained on the grounds — (1) that probably relatively few efferent visceral fibres 

 are given to the sympathetic from this region of the cord; (2) that many of the visceral efferent 

 fibres which do arise from this region of the cord probably join the rootlets of the spinal accessory 

 nerve and pass to the sympathetic system through the trunk of this nerve, and through the vagus 

 with which it anastomoses; and (3) that such of these fibres as are given off fi'om the lower 

 segments of the cervical region, descend the cord and pass out by way of the upper thoracic 

 nerves which give very evident white rami to the sympathetic. 



The meningeal or recurrent branch (figs. 747, 748, and 762) is very small and variable, 

 and is often difficult to find in ordinary dissections. It is given off from the nerve-trunk just 

 before its anterior and posterior primary divisions are formed. It consists of a few peripheral 

 branches of spinal ganghon-cells (sensory fibres) which leave the nerve-trunk and re-enter the 

 vertebral canal for the sensory innervation of the meninges, and which are joined by a twig 

 from the grey ramus or directly from the nearest 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 ganghon or sensory fibres 

 may turn backward 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.' Likewise, 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 

 primar}^ 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 pri- 

 marily a segmental 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 evi- 

 dent 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 over- 

 lap each other considerably during development. No nerve has a definitely pre- 

 scribed area of distri})ution, cutaneous or muscular, for its area is always consider- 

 ably overlapped b}^ the areas of the nerves adjacent to it. The mid-thoracic 

 nerves more nearly supply a definitely prescribed belt of the body. 



A. POSTERIOR PRIMARY DIVISIONS 



The ])ostorior ])rimary divisions of the spinal nerves spring from the trunks 

 immediately outside tlie intervertebral foramina, and they pass dorsalward 

 between the adjacent transverse processes. With the exceptions of the first and 

 second cervical nerves they are smaller than the corresponding anterior primary 

 divisions, which in these nerves is smaller from the fact that a large portion of 

 them go over into the hyi)oglossal or cervical loop. The posterior primary divi- 

 sions, after passing l)(!twcen tlie transverse processes into the region of the back, 

 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. 



