THE SPINAL NERVES 



811 



which there is a gray nodular enlargement, the spinal ganglion (Ganglion spinale). 

 Beyond the ganglion the dorsal root joins the ventral root to constitute the nerve. 

 The ganglia are external to the dura mater, and are situated in the intervertebral 

 foramina, except in the case of the sacral and coccygeal nerves, the ganglia of which 

 lie within the vertebral canal. Those of the coccygeal nerves are intradural. 



The ganglia vary greatly in size ; that of the first cervical nerve is scarcely as large as a hemp- 

 seed, while that of the eighth cervical is about 2 cm. long and 1 cm. wide. On the large roots 

 connected with the cervical and lumbar enlargement of the cord there are multiple ganglia of 

 varjdng sizes interposed in the course of the root-bundles. The fibers of the dorsal roots arise 

 from the cells of the spinal ganglia; connected with each ganglion cell there is a process which 

 bifurcates, giving rise to a fiber which enters the spinal cord and another which passes into the nerve. 



The ventral root (Radix ventralis) contains fewer fibers than the dorsal root, 

 except in the case of the first cervical nerve. It arises from the ventral surface of 

 the spinal cord (Fig. 626) by means of numerous small bundles of fibers which do 

 not form a linear series, but emerge from the cord over an area three to five 

 millimeters in width (ventral root zone). The fibers are processes of the 

 large cells of the ventral gray 



columns of the spinal cord. Spinal branch of intercostal artery 



There is no ganglion on the 

 ventral root. 



Spinal ganglion 



Dura mater 



Lig. denticulatum 



'-■- Dorsal root bundles 



Dorsal longitudinal ligament 



Conjugal ligament 



Fig. 651. — Vertebral Canal Opened by Sawing off the Arches. 

 (After Schm.iltz, Atlas d. Anat. d. Pferdes.) 



In the cervical, thoracic, and 

 anterior lumbar regions the bundles 

 of both roots pass through separate 

 openings in linear series in the dura 

 mater before uniting into a root 

 proper. Further back the bundles 

 of each root unite within the dura. 

 In the anterior part of the cervical 

 region and in the thoracic part of the 

 cord there are intervals of var3ang 

 length between adjacent roots, but in 

 some places the fibers of adjacent 

 roots overlap and an exchange of 

 fibers may be observed. Many of the 

 roots are directed almost straight out- 

 ward or incline slightly backward, but 

 the posterior lumbar, sacral, and coc- 

 cygeal roots and nerves run backward 

 to reach the foramina, through which 

 they emerge. The distance thus to 

 be traversed increases from before 

 backward, so that these nerves form 

 a tapering sheaf around the conus - • u ■ i iu 



meduUaris and filum terminale in the last lumbar vertebra and the sacrum, which is known as the 



Cauda equina. 



The size of the spinal nerves varies greatly. The largest are connected with 

 the cervical and lumbar enlargements. 



In, or immediately after its emergence from, the mtervertebral foramen each 

 spinal nerve gives off a small meningeal branch (Ramus meningeus). This is 

 ioined by a bundle of fibers from the ramus communicans and enters the vertebral 

 canal in which it is distributed. Each nerve then divides into two primary 

 branches, dorsal and ventral (Ramus dorsalis, ventralis). The dorsal branches 

 are smaller than the ventral, except in the cervical region. They are distributed 

 chiefly to the muscles and skin of the dorsal part of the body. The ventral branches 

 supply in general the muscles and skin of the ventral parts of the body, including 

 the limbs Each nerve or its ventral branch is connected with an adjacent ganglion 

 of the sympathetic system by at least one small short branch known as a ramus 

 communicans. Many nerves have two and some have three such rami. A nerve 

 may be connected with two ganglia, and a ganglion may be connected with two 

 nerves. 



