FORMATION OF NERVE-PLEXUSES 75 



central nervous system and ends in an autonomic ganglion — the 

 preganglionic fibre — while the other arises from a cell body in the 

 ganglion and ends in the organ to be controlled — the postganglionic 

 fibre. Although either a preganglionic fibre or a postganglionic 

 fibre may traverse several ganglia, the course between spinal cord 

 and periphery is usually interrupted in only one. The preganglionic 

 fibres, accompanied by visceral afferent elements, make up the 

 white communicating rami but the grey rami are composed of 

 postganglionic fibres destined to accompany the somatic branches 

 and end in the walls of the blood-vessels, in other smooth muscu- 

 lature, or in glands. 



The sympathetic trunk terminates anteriorly at the base of 

 the head in a relatively large superior cervical ganglion, which 

 receives its preganglionic fibres entirely from more posterior levels 

 by way of the cervical portion of the trunk. The second ganglion 

 in the rabbit is the inferior cervical, situated at the lower end of the 

 neck. From the inferior cervical and the first thoracic ganglia, 

 postganglionic fibres run as delicate grey rami along the vertebral 

 artery, forming a plexus about the latter and giving off a branch to 

 each cervical nerve as it crosses the artery. 



Plexus Formation 



In certain places, peripheral nerves, either spinal or autonomic, 

 connect with each other so as to form a plexus, or network. This 

 phenomenon is conspicuously exemplified by the nerves for each 

 of the limbs (brachial and lumbosacral plexuses). The develop- 

 ment of these limb-plexuses is probably an outcome of the manner 

 of origin of the limb-muscles, which involves the fusion of material 

 from the primary muscle-segments in the embryo and the sub- 

 sequent differentiation of the resulting mass into units which may 

 be derived from two or more segments. The originally segmented 

 nerves then become interconnected in such a way that each 

 definitive muscle will receive a nerve composed of the ap- 

 propriate number of fibres belongmg to each segment which has 

 contributed to its formation. The patterns of the plexuses are 

 subject to much individual variation. 



Observation of these and other pertinent facts has led to the 

 belief that the relation of nerves to their muscles is constant no 



