754 



THE NERVOUS SYSTEM. 



intermediate or collateral ganglia, on the branches or in the sympathetic plexuses ; 

 and terminal ganglia, in close relation to the endings of the nerves in the viscera. 



The nerve-fibres in the sympathetic system are of two classes, medullated and 

 non-medullated. The distinction is not absolute. The medullated fibres may lose 

 their medullary sheaths before reaching their terminations ; and the non-medullated 

 fibres may at their origin possess a medullary sheath. The medullated fibres form 

 the series of white rami communicantes (the visceral branches of the spinal nerves). 

 They take origin from the anterior rarni of certain spinal nerves in two streams: 

 thoracico-lumbar from the second thoracic to the second lumbar nerve inclusive, and 

 pelvic, or sacral, from the second and third, or third and fourth sacral nerves. The 

 roots of these rami arise from both posterior and anterior roots of the spinal nerves, 



but in largest numbers from the anterior root. 

 The fibres from the anterior root are of very 

 small size. They are the axons of nerve cells 

 within the spinal medulla, which enter the 

 sympathetic trunk through the white ramus, 

 and end by. forming arborisations around the 

 cells of a sympathetic ganglion. There are 

 three known courses for such a fibre to take in 

 relation to the sympathetic system (a) It 

 may end in the ganglion with which the ramus 

 is immediately related; (b) it may course up- 

 wards or downwards in the connecting cord to 

 reach a neighbouring ganglion ; (c) it may pass 

 beyond the sympathetic trunk to end in relation 

 to cells of the peripheral (collateral) ganglia 

 along with fibres of distribution from the sym- 

 pathetic ganglia. These fibres are splanchnic 

 efferent fibres ; motor fibres for the unstriped 

 muscular tissue of the vessels and viscera, and 

 secretory fibres for the glands in the splanchnic 

 area. The fibres from the posterior root of the 

 spinal nerve entering into the composition of 

 the white ramus communicans are the axons 

 FIG. 635. SCHEME OF THE CONSTITUTION OF of spinal ganglion cells. They constitute the 

 THE WHITE RAMUS COMMUNICANS OF THE splanchnic afferent fibres, and probably traverse 



the sympathetic gangliated trunk, passing up- 



The roots and trunks of a spinal nerve are war ds, downwards, and peripherally, without 

 tw^JespiLSrer^Sd'rj^^fhetid bein g connected with its cells. They are the 

 ganglion (Sy). The splanchnic efferent sensory fibres from the viscera, with which 

 fibres (in red) are shown, partly ending in they are associated along with the peripheral 



branches arising from the sympathetic trunk 

 (in blue) are itself. It is not certain that fibres from the 

 spinal ganglia are only found in connexion 

 with nerves provided with distinct white rami. 

 Similar medullated fibres are found also in the 

 gray rami communicantes. 



The non-medullated fibres in the sympathetic system are derived from the axons 

 of the sympathetic ganglion cells. They have different destinations, (a) Some 

 fibres appear to contribute to the formation of the cord connecting the 

 ganglia together, and to end in arborisations round the cells of a neighbouring 

 ganglion, (b) Non-medullated fibres form a large part of the system of peripheral 

 (splanchnic efferent) branches, streaming into the splanchnic area in an irregular 

 manner, both from the ganglia and the connecting cords. (c) The gray rami 

 communicantes form a series of non-medullated fibres (with a small number 

 of medullated fibres intermingled) proceeding centrally from the ganglia to the 

 spinal nerves. These gray rami are found in connexion with each and all of the 

 spinal nerves. Their origin from the gangliated trunk is quite irregular : they may 

 come from the ganglia or the commissure; they may divide after their origin, so 



SPLANCHNIC 

 EFFERENT 



splanchnic afferent fibres n ue are 



shown, partly entering the ganglion, and 



cord into peripheral branches. 



