686 



THE NERVOUS SYSTEM. 



laterally in the vertebral canal towards the intervertebral foramina, where they 

 unite to form the spinal nerve. The direction of the roots of the first two nerves 

 is upwards and laterally ; the roots of the remaining nerves course obliquely 

 downwards and laterally, the obliquity gradually increasing until, in the case of 

 the lower lumbar, the sacral and coccygeal nerve-roots, their course is vertically 

 downwards in the vertebral canal. The collection of nerve -roots which occupies 

 the lower part of the canal, below the first lumbar vertebra, and comprises all the 

 nerve-roots below those of the first lumbar nerve, is designated the cauda equina. 

 They arise from the lumbar enlargement and conns medullaris, and surround the 

 tilum terminale of the spinal medulla. 



Within the vertebral canal the nerve-roots are in relation with the meninges of 

 the spinal medulla, and are separated from one another by the ligamentum denticu- 

 latum, and, in the neck, by the spinal part of the accessory nerve. Each receives a 

 covering of pia mater, continuous with the neurilemma; the arachnoid invests 



Posterior column of spinal medulla 



Posterior nerve-root 

 Anterior nerve-root | 



Anterior column of spinal medulln 



Spinal ganglio 

 Posterior ramus (medial branch > 



Posterior rani 



Posterior ramus (lateral branch; 

 Recurrent meniiigeal branch (uniting with a sympathetic branch 



Gray ramus cominunicans 



Splanchnic branch (white ramus 



cominunicans) 



Anterior raiuu 



Lateral branch (posterior 

 subdivision) 



Lateral branch 



Anterior branch 



Lateral branch (anterior^* 

 subdivision) ~"1 



Gangliated sympathetic trunk 

 Efferent (vaso-motor) branch 



Aorta 

 Cardinal vein 



Afferent viscero-iuhibitory 

 branch 



FIG. 606. THE ORIGIN AND DISTRIBUTION OF A TYPICAL SPINAL NERVE. 



each root as far as the point where it meets with the dura mater ; and each root 

 pierces the dura mater separately. The two roots are thereafter enclosed in a 

 single tubular sheath of dura mater, in which is included the spinal ganglion of 

 the posterior root. The spinal nerve thus ensheathed occupies the intervertebral 

 foramen (except the first two cervical and the sacral and coccygeal nerves). 



Divisions of a Spinal Nerve. After emerging from the intervertebral foramen 

 the nerve immediately divides into two primary divisions, named respectively 

 the posterior and anterior rami (O.T. posterior and anterior primary divisions). Just 

 before its division each nerve gives off a minute rneningeal (recurrent) branch, 

 which re-enters the vertebral canal after effecting a junction with a branch from 

 the sympathetic trunk, and is distributed to the spinal medulla and its membranes. 



The posterior and anterior rami of the spinal nerves are mainly somatic in their 

 distribution, and are responsible for the innervation of the skeletal muscles and 

 of the skin covering the trunk and limbs. 



