THE SYMTATIIKTHJ XYXTKM 1001 



neurones may ho involved in tho transmission of a {riven impulse when sent from a 

 region <listant from the blague to which it is distributed. 



Communication between the central nervous system and the sympathetic is 

 established through both efferent and afferent lilires. In tlic region of tiie spinal cord 

 both varieties of fibres p : ,-s from one system to the other by way of the rami com- 

 municantes, delicate bundles of fibres connecting the nearby sympathetic trunk with 

 the respective? spinal nerve-. The efferent fibres of the rami arise m the ventral horn 

 of the Bpinal cord, emerge through the ventral mots, enter the rami. and terminate for 

 the most part about the cells cf the nearest sympathetic ganglion; some, however, 

 may pass through or over the ganglion of the sympathetic cord and terminate about 

 cells in more distant ganglia. Since these fibres transmit impulses from the central 

 to the sympathetic system, they are known as efferent splanchnic fibres. They 

 are of smaller size than is the average for the cerebro-spinal efferent or motor fibres 

 of the ventral root. The afferent splanchnic fibres are of two varieties: (1) Per- 

 ipheral processes of the spinal ganglion-cells which run outwards in the nerve-trunk, 

 enter the rami communicantes, pass through the various connecting trunks and ter- 

 minal rami of the sympathetic and terminate in the tissues supplied by these rami. 

 Such are merely sensory fibres of the cerebro-spinal type which collect impulses in 

 the domain of the sympathetic and convey them to the central system by way of the 

 sympathetic nerves and the dorsal roots of the spinal nerves. (2) Afferent sympa- 

 thetic fibres proper. The actual existence of these has not been long established, and 

 their relative abundance is as yet uncertain. They consist of fibres arising in the 

 sympathetic ganglia which enter the spinal ganglia by way of the rami communicantes 

 and the cerebro-spinal nerve-trunk and terminate in arborisations about the spinal 

 ganglion-cells, chiefly about the Dogiel cells of type II (D, fig. 717). The afferent 

 impulses transmitted by these sympathetic fibres are borne into the spinal cord by 

 way of the cerebro-spinal fibres of the dorsal mots. They must necessarily either 

 receive the impulses they bear from sympathetic neurones having both peripheral 

 and central processes or they themselves must be axones or central processes of neu- 

 rones having also processes terminating in the peripheral tissues. 



The thoracic and the lumbar spinal nerves are connected with the sympathetic 

 trunk (gangliated cord) by two rami communicantes. Most of both the efferent 

 splanchnic and also the afferent splanchnic fibres (which arise in the spinal ganglia) 

 pass by way of a separate ramus. Both these varieties being of the cerebro-spinal 

 type, and, therefore, medullated, they give the ramus a white appearance meriting 

 the name white ramus communicans. Fibres of the sympathetic type predomi- 

 nate in the second ramus and thus it is the grey ramus communicans. The latter 

 consists of: (1) afferent sympathetic fibres and (2) of sympathetic fibres which 

 join the primary divisions of the spinal nerves and course in them to their allotted 

 tissues (fig. 717). In the sacral region, most of the efferent splanchnic fibres pass 

 over the ganglia of the sympathetic trunk and terminate in the more peripheral ganglia 

 of the plexuses of this region. This is especially true for the fibres passing from the 

 second, third, and fourth sacral nerves. In the cervical region white rami are not 

 in evidence, a fact probably explicable as due to an arrangement by which at least 

 most of the efferent splanchnic fibres arising in the cervical segments of the spinal 

 cord pass downwards in these segments and join the sympathetic through the white 

 rami of the upper thoracic nerves; others may enter the cervical portion of the 

 gangliated cord through the spinal accessory or eleventh cranial nerve, rather than 

 through individual white rami. All the spinal nerves are joined by grey rami com- 

 municantes from the sympathetic trunk. 



\ a so-motor fibres to the meninges and intrinsic blood-vessels of the spinal cord 

 pass to the spinal nerves by way of the grey rami. Thence they may reach the men- 

 iiiires by one of three ways: (i) through the delicate recurrent or meningeal branch 

 of the f spinal nerve (fig. 717); (2) through the trunk and ventral root of the spinal 

 nerve; (3) probably more rarely, through the trunk and dorsal root of the spinal 

 nerve (fig. 7 IS). 



Corresponding communications exist between the cranial nerves and the sym- 

 pathetic, but these occur further towards the periphery and in not so regular a 

 manner as the communications between the spinal nerves and the sympathetic 

 system. The mesencephalon. for example, is chiefly connected with the ciliary 

 ganglion of the sympathetic by fibres which are sent through the oculo-motor nerve 

 and which enter this ganglion by way of its short root and terminate about its cells. 



