366 THE PERIPHERAL NERVOUS SYSTEM 



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 

 comparable to thlat 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 cerebral 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 cerebral nerves 

 furnish unreliable evidence as to the primitive number of cephalic 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 is also known as the 

 autonomic system, for it has a certain degree of independence of the central 

 nervous system. 



The sympathetic ganglion cells are^derived from the cells of the gan- 

 glion crest. In fishes, discrete cellular masses become detached from the 

 spinal ganglia. At an early stage (6 to 7 mm.) in human development, on 

 the contrary, certain cells of the ganglion crest (and neural tube; Kuntz, 

 1910) migrate ventrally along the nerve roots and give rise to a series of 

 ganglia, which, in the region of the trunk, are segmentally arranged (Fig. 

 360). 



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 posi- 

 tion lateral to the aorta (Fig. 325). These sympathetic anlages are at 

 first distinct, but at 9 mm. unite with each other from segment to seg- 

 ment, forming a longitudinal ganglionated cord. After the formation of 

 the primitive rami communicaates by 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 mye- 

 linated and constitute the white rami; the sympathetic, centripetal fibers 

 remain unmyelinated and form separately the gray rami. Nerve fibers 

 appear in the paired longitudinal cords, which were at first purely cellular, 

 in such a manner that segmental masses of cells (sympathetic ganglia) be- 

 come linked by fibrous, commissural cords. The more peripheral ganglia 

 (cardiac and cceliac) and the sympathetic ganglia of the head may be found 

 in 1 6 mm. embryos (Fig. 366). 



In the head region the sympathetic ganglia are not segmentally ar- 

 ranged, but are derived from cells of the cerebrospinal ganglia that mi- 

 grate to a ventral position (Fig. 365). These cells likewise give rise to nerve 



