7(i THE NERVOUS SYSTEM 



SPINAL NERVES 



The spinal nerves s])rinjf t'loni the spinal cord l)v four long series of roots, 

 namely, an anterior and a ])osterior series on each side of the cord. These origins 

 correspond with the superficial origins of the cranial nerves, l)eing the points 

 at which the nt-rve-tihres emerge from the cere])ro-spinal axis. Like the cranial 

 nerves, the spinal nerves have also a deep origin. This is situated in the anterior 

 and posterior horns of the grey crescents and has heen already described (i)age 738). 

 The spinal nerves also resemble the cranial nerves, inasmuch as this ' deep ' origin 

 is only a preliminary interruption in nerve-cells or in plexuses in the grey matter, 

 and from the deep origin tracts of fibres pass ujiwardsto establish a connection Avith 

 the cere])ral cortex. They differ from the cranial nerves inasmuch as they arise 

 from the spinal cord instead of from the encephalon, in their mode of origin l)y an 

 anterior motor and posterior sensory root, and by the development, on each of tlu^ 

 posterior roots, of a ganglion (the spinal ganglion). Certain of the cranial nerves 

 resemble the spinal nerves in the two latter respects; for example, the vagus arises 

 from the large-celled (motor nucleus), a derivative of the anterior horn, and from 

 the small-celled (sensory nucleus) derived from the basal part of the posterior 

 horn; the roots from these two sources have, however, united before the nerve 

 has reached the surface of the medulla. The ganglion of the root of the vagus 

 and the ganglion on the sensory root of the trigeminal (Gasserian ganglion) are 

 clearly homologous to the ganglia on the posterior roots of the spinal nerves. The 

 hyjioglossal nerve is, in rare cases, joined by a small posterior root on which a 

 ganglion is developed, and, on the other hand, cases are recorded in Avhich the 

 posterior root of the first spinal nerve was devoid of a ganglion. 



The anterior roots stand out in marked contrast to the posterior roots in regard 

 to their function, being motor, whereas the posterior roots are sensor}'. They also 

 differ from the posterior roots in several anatomical points. The anterior roots 

 (with the single exception of the first) are smaller than the posterior; they arise by 

 four to six fasciculi, and their origins do not form a linear series, but map out a 

 longitudinal area of one to tAvo millimetres in breadth on the surface of the cord. 

 The posterior roots are larger than the anterior, they arise by six to eight fasciculi, 

 are ])laeed in a strictly linear series, and, as each I'oot is traversing the corivs- 

 ponding intervertebral foramen, it enters into a sjiinal ganglion, a structure with 

 Avhich the motor root has no connection A^hatever. 



The posterior roots are from one and a half to three times as large as the 

 anterior, the ratio being — 



1 to 1*5 in the thoracic nerves. 1 to 2 in the lumbar nerves. 



1 to 3 in the cervical nerves. 



Course and direction. — From their superficial origin, both anterior and 

 posterior roots proceed towards the intervertebral foramina, and unite near the 

 outer limits of the foramina into single trunks. The ganglia on the posterior roots 

 are placed, in the case of the majority of the nerves, within the foramina inune- 

 diately internal to the point of junction of the tAvo roots. The ganglia of the first 

 and second cervical nerA'es are placed on the laminae of the atlas and axis. The 

 ganglia of the sacral and coccygeal nerves are placed Avithin tire sj^inal canal. Each 

 spinal ganglion is an ovoid greyish ])ody, the long axis of Avhich corresponds to the 

 axis of the nerve Avith Avhich it is incorporated. It is someAvhat adherent to its 

 meningeal sheath. 



The roots of the first spinal nerve ascend slightly to reach the interval between 

 the atlas and the occipital bone. The second and third nerves pass horizontally 

 outAvards, the fourth passes obliquely doAvuAvards and outAvards, and the remaining 

 nerves pass out Avith increasing degrees of obliquity, the intraspinal course of the 

 nerve-roots increasing in length as the series is folloAved doAVUAvards. It folloAVs 

 from the above statement that the loAver nerve-roots are directed almost vertically 

 dowuAvards, and as the si)inal cord ends at the level of the second lumbar vertebra, 

 Avhile the series of intervertebral foramina is continued to the lower end of the 



