366 THE PERIPHERAL NERVOUS SYSTEM 



the branchial arches (branchiomerism) , and by the discovery, in the embryos of lower ver- 

 tebrates, of so-called head cavities, homologous with mesodermal segments. (Note also the 

 presence of neuromeres, p. 334.) 



Assuming that the branchiomeres are portions of the primary head segments — and 

 there are recent observations which tend to disprove this — their segmentation is still not com- 

 parable to that of the trunk, for the branchial arches are formed by the segmentation of 

 splanchnic mesoderm, tissue which in the trunk never segments. The branchial arches, 

 therefore, represent a different sort of metamerism. 



Only the first three head cavities persist. These form the eye muscles, innervated by 

 the third, fourth, and sixth cranial nerves respectively. All the remaining muscles of the 

 head are derived from the branchiomeres. From what has been said it is evident that one 

 cannot compare the relation of the cranial nerves to the branchiomeric muscles with the 

 relation of a spinal nerve to its myotomic muscles. For this reason the cranial nerves furnish 

 unreliable evidence as to the primitive number of cephahc segments. Various investigators 

 have set this number between eight and nineteen. 



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 fimction 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 ganghon 



crest. In fishes discrete cellular masses become detached from the spinal gangha. 



At an early stage (6 to 7 mm.) in human development, on the contrary, certain 



cells of the ganghon crest migrate ventrally and give rise to a series of ganglia, 



which, in the region of the trunk, are segmen tally arranged (Figs. 139 and 360). 



According to Kuntz (Jour. Comp. Neurol, vol. 20, 1910), the primary source of 



these errant cells is the neural tube, from which they migrate along both dorsal 



and ventral nerve roots. At 9 mm. the ganglionated cord is formed and fibers 



connecting the sympathetic gangha with the spinal nerves constitute the rami 



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



and the sympathetic ganglia of the head may be found in 16 mm. embryos (Fig. 

 366). 



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

 ventrally in advance of the ventral root fibers and take up a position lateral to the 

 aorta. These ganglionic anlages are at first distinct, but soon unite with each 

 other from segment to segment, forming a longitudinal cord of cells. After the 

 formation of the primitive rami communicantes by the root fibers from the spinal 

 nerves, centripetal processes from the sjonpathetic cells grow back and join the 

 trunks of the spinal nerves. The visceral, spinal fibers later become myelinated 

 and constitute the white rami; the sympathetic, centripetal fibers remain unmye- 



