Chap. XIX] THE NERVOUS SYSTEM 377 



tion of this name, the fourth group was not included in the sym- 

 pathetic system. The term autonomic impHes that these nerves 

 are to some extent independent of the central nervous system and 

 possess a certain amount of self-government. 



Rami communicantes. — The nerve-fibres that connect the 

 vertebral ganglia and the spinal nerves are called rami communi- 

 cantes. Each connection consists of two rami, one white and the 

 other gray. The white ramus consists of medullated fibres, and 

 these pass from the cord to the ganglion. The gray ramus consists 

 of non-medullated fibres that pass from the ganglion to join the 

 spinal nerve. 



Plexuses. — The term plexus has been used to designate a net- 

 work of nerves. It is worthy of special mention because the nerve- 

 fibres arborize with each other, and there is an interchange of fibres 

 between the different nerve-trunks. The advantages of this ar- 

 rangement are : (1) each nerve is less dependent on the unimpaired 

 condition of any single portion of the nerve-trunk or nerve-centre, 



(2) each nerve has a wider communication with the nerve-centres, 

 and (3) any given part of the body is not dependent on one nerve. 

 The various plexuses of the sympathetic system serve all these 

 purposes, and in addition the organs constituting any one system 

 are brought into direct communication with each other. In this 

 way coordination of action is secured. 



Distribution of sympathetic nerves. — Nerve-fibres from the 

 sympathetic system are distributed: (1) to the heart, (2) to the 

 involuntary muscles of the blood-vessels, lymphatics, and viscera, 



(3) to the secretory glands, and (4) to some of the special senses, 

 such as those that regulate the pupil of the eye. 



Interdependence of the sympathetic and central nervous sys- 

 tems. — The fibres which connect the s^^mpathetic ganglia and the 

 spinal nerves form a direct pathway for impulses from all of the 

 viscera to the spinal cord and brain. In addition many of the 

 viscera are connected with the brain by the cranial nerves (see 

 page 395). This means that there are often two sets of nerve-fibres 

 distributed to an organ, and we know that in some instances the 

 action of these fibres is antagonistic. This is true in the case of the 

 inhibitory and accelerator fibres of the heart, and increased knowl- 

 edge may prove it to be true in connection with all the organs of the 

 body. 



