Chap. XIX] 



THE NERVOUS SYSTEM 



375 



tres are situated in the medulla and spinal cord. Most of the cen- 

 tres in the medulla are concerned with processes that are absolutely 

 necessary to life, hence are called vital centres. Examples of these 

 are the centres controlling respiration and the cardiac centre. 



Intercentral neurones and relays. — Neurones whose processes 

 do not pass outside of the brain or spinal cord are called intercentral 

 or connecting neurones. An impulse passing from say the foot 

 to the brain, might have to pass through two or three, or even 



Fig. 179. — Transverse Section of the Sciatic Nerve of Cat about X 100. 

 — It consists of bundles (funiculi) of nerve-fibers ensheathed in a fibrous supporting 

 capsule. 



more, neurones ; these neurones form a system of relays. A par- 

 allel may be found in sending a telegraphic message ; the message 

 may have to pass over several different systems of wires and even- 

 tually be carried by messenger before it reaches its destination. 

 These relays only occur in the brain and spinal cord. In the illus- 

 tration referred to above, one relay would take the impulse to the 

 spinal cord ; this naturally means a very long fibre ; one or several 

 might be required to take it from the spinal cord to the brain. 



THE SYMPATHETIC SYSTEM 



The sympathetic system consists of three sets of ganglia and the 

 nerves connected with them : — 



1. Vertebral or lateral ganglia. 



2. Collateral or prevertebral ganglia, and plexuses. 



3. Terminal ganglia and plexuses. 



4. Sympathetic ganglia in the brain and cord. 



