SPINAL NERVES. 



439 



Fig. 282. 



1. The Spinal Nerves. 



The thirty-one pairs of nerves connected with the spinal cord 

 rise from two roots, anterior and posterior, or motor and sensory. 

 (Fig. 282.) Sometimes, how- 

 ever, the first nerve has only 

 an anterior root, and the last 

 a posterior only. 



1. The roots of the spinal 

 nerves are inclosed in a de- 

 licate perineurium derived 

 from the pia mater, g^Vtf of 

 an inch thick, and sending in- 

 ternal septa among the indi- 

 vidual nerve-fibres, which are 

 coarse in the motor root. An- 

 astomoses between the roots 

 frequently occur, and far most 

 so between the posterior roots. 



The two roots converging, 

 separately perforate the arach- 

 noid and dura mater, and re- 

 ceive a firmer investment 

 from the latter; then proceed- 

 ing further, the posterior root 

 forms its ganglion; after which 

 the two roots unite to form 

 the common spinal nerve- 

 trunk. 



2. The ganglion on the pos- 

 terior root is formed as fol- 

 lows: Nerve-cells (ganglion-cells) are developed among, but princi- 

 pally externally to, the nerve-fibres, each giving off one, two, or 

 several nerve-fibres (ganglion-fibres), which proceed in company with 

 the original sensitive fibres of the posterior root, and merely pass 

 among the ganglion-cells, and reunite after traversing the ganglion. 

 The root, however, becomes somewhat increased in size from the 

 addition of the ganglion-fibres. (Fig. 283.) 



The ganglion-cells have a distinct membrane, are mostly rounded, 

 elongated, or pyriform, and a little flattened, averaging * to 



The two roots of a dorsal spinal nerve, and its 

 union with the sympathetic: c, c. Anterior fissure of 

 the spinal cord. a. Anterior root. p. Posterior, with 

 its ganglion, a'. Anterior branch, p'. Posterior branch. 

 8. Sympathetic, e. Its double junction with the an- 

 terior branch of the spinal nerve by a white and a 

 gray filament. 



