DEVELOPMENT OF THE SYMPATHETIC SYSTEM. 



681 



trunks are homologous with the lateral and ventral branches of the somatic nerves in 

 other regions. 



DEVELOPMENT OF THE SYMPATHETIC SYSTEM. 



There are two conflicting views of the mode of development of the sympathetic 

 system. 



In birds and mammals the first rudiment of the sympathetic trunk occurs in the 

 formation of a longitudinal unsegmented column of mesodermic cells (which stain more 

 deeply than the mesoderm in which they lie) on each side of the aorta, and coterminous 

 with it. This column of cells becomes joined at an early stage by the visceral branches 

 of the spinal nerves which grow inwards from the main nerve trunks into the splanchnic area, 

 and result from the division of the nerve into 

 somatic and visceral parts. These visceral 

 branches constitute the white rami communi- 

 cantes. They receive contributions usually from 

 both posterior and anterior roots, and gradually 

 approaching the above-mentioned column of 

 mesodermic cells, they become intimately associ- 

 ated with the cells. In some cases fibres of the 

 visceral nerves pass over the cellular column 

 into the splanchnic area without connexion with 

 it (Fig. 603). By the junction of these visceral 

 nerves with the cells of the column, certain 

 cells persist and produce the ganglia. The in- 

 tervening portions of the column, by changes in 

 the cells, and by the addition of fibres belonging 

 to the visceral nerves, give rise to the con- 

 necting cords. The cellular column, besides 

 producing the gangliated trunk, by the further 

 growth of its cells and their extension centrally 

 and peripherally, produces the gray rami com- 

 municantes, parts of the peripheral branches, 

 and the peripheral (collateral and terminal) 

 ganglia, as well as the medullary portion of the 

 suprarenal gland. The cervical, lower lumbar, 

 and sacral portions of the sympathetic gangliated 

 trunk are secondary extensions from the primitive 

 trunk, gradually growing upwards and downwards 

 along the main vessels. These portions of the 

 system are not provided with white rami com- 

 municantes. The ganglia of the sympathetic 

 assume their segmented appearance (1) from the 

 persistence of the primitive cells and their con- 

 nexion with the spinal nerves by means of the 

 white and gray rami communicantes, and (2) 

 from the way in which the primitive column is Sy, Sympathetic trunk ; Spl, Splanchnic branchy 

 , , , , J , , . of spinal nerves (white rami communi- 



moulded by the surrounding structures (bones, cantes) ; V.S, Vertebral segments; D.G, 



segmental arteries, etc.). Spinal ganglia. 



In another account of the development of 



the sympathetic system (Onodi), the gangliated trunk is described as an outgrowth of 

 the thoracic spinal ganglia of the spinal nerves. It is said that each ganglion gives off 

 a bud at its inferior end, which, growing inwards into the splanchnic area, becomes 

 attached to the trunk of the spinal nerve just beyond the union of the posterior and 

 anterior roots. The bud still extending inwards into the splanchnic area, remains 

 associated with the nerve by an attenuated stalk. These buds, it is said, become the 

 ganglia, which, after reaching their permanent place in the splanchnic area, are sup- 

 posed to grow upwards and downwards so as to coalesce and form a beaded chain of 

 ganglia. The stalks connecting the ganglia with the spinal nerves become the white 

 rami communicantes. This mode of development does not satisfactorily account for 

 several important features of the sympathetic system the development of those parts 

 of the gangliated trunk which possess no white rami, the absence of a truly segmental 

 character in the trunk, and the constancy of its continuity. No instance is recorded 



FIG. 603. THE DEVELOPMENT OF THE 

 SYMPATHETIC GANGLIATED TRUNK. 



