968 



THE NERVOUS SYSTEM 



In the upper half of the spinal nerves the medial branches supply the skin; in the lower half, 

 it is the lateral branches which do so. Both branches of almost all the posterior divisions, espe- 

 cially those of the lower nerves, show a tendency to run caudalward and thus are distributed 

 to muscles and skin below the levels of their respective intervertebral foramina. They never 

 supplj' the muscles of the limbs, though their cutaneous distribution extends upon the buttock, 

 the shoulder, and the skin of the back of the head as far upward as the vertex. The posterior 

 primary divisions, with the exception of those of the first three cervical nerves, are much smaller 

 than the anterior primary divisions. 



As their mixed function suggests, the posterior primary divisions contain both nerve-fibres 

 from the ventral roots and peripheral processes of the spinal ganghon-cells. If the nerve-trunk 

 on the immediate peripheral side of the spinal gangUon be teased, bundles of ventral root-fibres 

 may be seen crossing the trunk obhquely to enter the posterior division, and fibres from the 

 spinal gangUon may be also traced into it. Also a few sympathetic fibres, derived chiefly by 

 way of the ramus communicans, are known to course in it for distribution in the walls of the 

 blood-vessels, etc., of the area it supphes. 



The anterior primary divisions run lateralward and ventralward. With the 

 exception of the first two cervical nerves, which contribute the hypoglossal loop, 

 they are larger than the posterior primary divisions, and appear as direct continu- 

 ations of the nerve-trunks. Only in case of most of the thoracic nerves do they 

 remain independent in their course. In these they run lateralward and ventral- 

 ward in the body-wall. In general, these divisions supply the lateral and ventral 



Fig. 



748. — Diagram Illustrating the Origin of the Component Nerve-fibres of the 

 Primary Divisions of a Typical Spinal Nerve. 



Spinal ganglion neurone 

 to capsule of ganglion 



Dorso-lateral group of 

 ventral horn cells 



Gray ramus communicans 

 White ramus communicans 



Sympathetic ganglion 1 Gangliated 

 Sympathetic trunk / trunk 

 Sympathetic cell body in spinal 

 ganglion 



Posterior primary division 1 Spinal 

 Anterior primary division J nerve 



Afferent spinal fibre 

 ^ Efferent visceral fibre 



^^^ ^Gray ramus communicans 



** ^White ramus communicans 



Sensory sympathetic neurone 

 Branch to prevertebral ganglion 



parts of the body, the limbs, and the perineum. In the cervical, lumbar, and 

 sacral regions they lose their anatomical identity by dividing, subdividing, and 

 anastomosing with each other so as to give rise to the three great spinal plexuses of 

 the body — the cervical, the brachial, and the lumbo-sacral plexuses. The major- 

 ity of the thoracic nerves retain the tj'pical or primitive character in both their 

 anterior and posterior primary divisions. In them the anterior division (inter- 

 costal nerve) divides into a lateral or dorsal and an anterior or ventral branch, 

 both of whicli sul)dividc. The lateral branch is chiefly cutaneous; it pierces the 

 superficial muscles and, in the subcutaneous connective tissue, divides into a 

 smaller posterior and a larger anterior ramus, which respectively supply the skin 

 of the sides and tlie lateral part of the ventral surface of the body. The anterior 

 branch continues ventralward in the body-wall, giving off twigs along its course to 

 the adjacent muscles and bones, and, as it approaches the ventral mid-line of the 

 body, it turns sharply lateralward and sends rami medialward and lateralward to 

 supply the skin of the ventral asi)ect of the body. In the region of the limbs the 

 typical arrangement is interfered with in that what corresponds to the lateral and 

 anterior branches of the division are carried out into the limbs for the skin and 

 muscles there, instead of supplying the lateral and ventral parts of the body-wall. 



