364 THE PERIPHERAL NERVOUS SYSTEM 



and nodose ganglia. The nervous elements supplying these vestigial organs have completely 

 disappeared. 



C. THE SYMPATHETIC NERVOUS SYSTEM 

 The sympathetic nervous system is composed of a series of ganglia and 

 peripheral nerves, the fibers of which supply gland cells and the smooth muscle 

 fibers of the viscera and blood-vessels. It may function independently of the 

 central nervous system and is hence known as the autonomic system. 



The sympathetic ganglion cells are derived from the cells of the ganglion 

 crest. At an early stage (6 to 7 mm.) certain cells of this crest migrate ventrally 

 and give rise to a series of ganglia which, in the region of the trunk, are segmentally 

 arranged (Fig. 342). The migration of the sympathetic cells is rapidly taking 

 place in embryos of 6 to 7 mm. At 9 mm. the ganglionated cord is formed and 

 fibers connecting the sympathetic ganglia with the spinal nerves constitute the 

 rami communicantes (Streeter). The more peripheral ganglia (cardiac and 

 coeliac) and the sympathetic ganglia of the head may be found in 16 mm. embryos 



(Fig. 347)- 



The cells which are to form the ganglia of the sympathetic chain migrate 

 ventrally ahead of the efferent fibers and take up a position lateral to the aorta. 

 The ganglionic anlages are at first distinct but unite with each other from segment 

 to segment, forming a longitudinal cord of cells. After the formation of the rami 

 communicantes by the root fibers from the spinal nerves centripetal processes 

 from the sympathetic cells grow back and join the trunks of the spinal nerves. 

 The visceral spinal fibers later become medullated and constitute the white rami; 

 the sympathetic centripetal fibers remain non-medullated and form the gray rami 

 of each ramus communicans (Fig. 342). From neurones of the ganglionated 

 cord nerve fibers extend from ganglion to ganglion and thus the cellular cord is in 

 part converted into a fibrous longitudinal commissure. This commissure con- 

 nects the persisting cellular masses which constitute the sympathetic ganglia of 

 each segment. In the head region the sympathetic ganglia are not segmentally 

 arranged but they are derived from cells of the cerebrospinal ganglia which 

 migrate to a ventral position (Fig. 346). These cells likewise give rise to nerve 

 fibers which constitute longitudinal commissures connecting the various ganglia 

 of the head with the ganglionated cord of the trunk region. The small cranial 

 sympathetic ganglia are probably all derived from the anlage of the semilunar 

 ganglion (Fig. 347). The ciliary ganglion is related by a ramus communicans to 

 the ophthalmic division of the trigeminal nerve and receives fibers from the 



