616 MORPHOLOGY OF A SPINAL NERVE. 



3), which is occasionally double on the lumbar and sacral nerves. Beyond _ the ganglion, the 

 two roots unite to form within the spinal canal the mixed trunk of a spinal nerve. The 

 branches of the nerve-trunk invariably contain fibres coming from both roots. The number of 

 fibres in the nerve-trunk is exactly the same as in the two roots ; hence, we must conclude that 

 the nerve-cells in the spinal ganglion are intercalated in the course of the fibres (Gaule and 



Varieties. The spinal ganglion is sometimes double, and according to Hyrtl, isolated 

 ganglionic cells frequently occur in the posterior root, between the ganglion and the cord. 

 Occasionally the roots are somewhat unsymmetrical on opposite sides ; in the dorsal part one 

 or other, or both roots of a spinal nerve are sometimes absent (Adamkiewicz). 



[Morphology of the Spinal Nerves and Limb-Plexuses. A typical segmental spinal nerve 

 (fig. 435) divides, after its formation, into three parts, a dorsal branch, or superior primary 



division, distributed to the back, a soma- 

 tic branch, or inferior primary division, 

 supplying the body-wall or limbs ; and 

 a splanchnic or visceral branch, or ramus 

 communicans, connected with the sym- 

 pathetic gangliated cord, and distributed 

 to the large vessels and viscera. The 

 somatic branch is the largest, and is 

 generally, by human anatomists, spoken 

 of as the "anterior primary division." 

 In the thoracic and upper lumbar regions, 

 the distribution of this nerve is simple. 

 It divides into an external (or lateral) 

 branch, and an internal (or anterior) 

 branch, which supply respectively the 

 lateral and anterior portions of the tho- 

 racic and abdominal walls.] 



[In the region of the neck, and in re- 

 lation to the limbs, the arrangement of 

 the somatic branches becomes complicated 

 by the formation of the plexuses. In 

 Fig. 435. * ne emDr y> however, the distribution of 



-.. f . , ~" . , , the nerves is simpler, and a comparison 



Diagram of a spinal nerve ; C, spinal cord ; pr, ar, be ^ both ^ the adult a { _ 



posterior and anterior roots ; SPD IPD superior ment , and with the typical nerve as seen 

 and inferior primary divisions ; d, v dorsal and fa h ; h . ^ In the emb 



ventral branches ; sr, sympathetic root (Boss). the ^ ag guch J^ not ^ ftud he 



upper limb sprouts out directly beyond the segmented visceral arches. In this state, the 

 somatic branch if distributed as in the thoracic region ; the nerve divides into an external and 

 an internal branch, distributed to the side and front of the corresponding part of the arches 

 in the neck, in the regions where the limbs are appearing as two flattened buds from the ventro- 

 lateral aspect of the body. The somatic branch sweeps round into the blastema forming the 

 limb, and divides into its two branches, external and internal, or dorsal and ventral, which 

 are distributed to the outer (dorsal) and inner (ventral) surfaces, respectively, of the primitive 

 limb. At this time, the cartilaginous and muscular elements of the limb have not become 

 differentiated. While this is occurring, the dorsal and ventral parts of the somatic branches 

 of the nerves entering the limb unite with adjacent dorsal and ventral branches, in various com- 

 binations, so as to produce the limb-plexuses. The nerves resulting from these combinations 

 are distributed to the primitive, dorsal, and ventral surfaces of the limbs. Thus, the plexuses 

 are formed, and the peripheral distribution of the nerves has taken place before the period of 

 flexion and angulation of the limbs. These processes mark the conditions in the adult ; but 

 even then it is easy to make out that the nerves in the upper limb derived from the posterior 

 (dorsal) cords of the brachial plexus supply the scapular region, extensor surface of the arm 

 and fore-arm, and the back of the hand, parts which are derived from the dorsal surface o,f 

 the primitive limb ; while the nerves produced from the anterior (ventral) cords supply the 

 pectoral region, front of the arm, fore-arm, and hand, parts representing the primitive ventral 

 surface. ] 



[In the lower limb, the nerves derived from a union of the posterior branches are the external 

 cutaneous, anterior crural, gluteal, and external popliteal. These supply the iliac surfaces, the 

 front of the thigh, leg, and foot, belonging to the primitive dorsal surface of the limb. 

 The nerves formed by the union of anterior branches, genitocrural, obturator, and internal 

 popliteal, in like manner supply the parts of the limb corresponding to the ventral surface, 

 the inner side and back of the thigh, the back of the leg, and the sole of the foot (A. M. 

 Paterson). ] 



[Structure of a Spinal Ganglion. The ganglion is invested by a thin, firmly adherent, sheath 



