466 - Multicellular Animals, Especially Man 



,CRANIAL NERVE 



PARASYMPATHETIC 

 SYSTEM 



PARASYMPATHETIC GANGLIA 



^©"T " EYE 



SPINAL CORD 



SYMPATHETIC 

 SYSTEM 



PARASYMPATHETIC 

 SYSTEM 



PANCREAS 



- LARGE 

 INTESTINE 



Fig. 25-16. Distribution and connections of the autonomic nervous system. The heavy lines 

 represent preganglionic fibers; the dotted lines, postganglionic fibers. 



heartbeat, or the peristaltic movements of 

 the gastrointestinal tract, although regulated 

 by reflexes, cannot be controlled on a voli- 

 tional basis. 



One peculiar feature of the autonomic 

 nerves lies in the fact that motor impulses, 

 leaving the spinal cord or brain, reach the 

 effector organ, not by way of a single neuron, 

 as in the case of the skeletal muscles, but 

 always by a relay of tiuo neurons. In every 



case the autonomic nerve is interrupted b\ 

 a motor ganglion in which the preganglionic 

 neurons establish synaptic connections with 

 a second relay of postganglionic neurons 

 (Fig. 25-16). Each preganglionic neuron trans- 

 mits its impulses to several or many post- 

 ganglionic nerve cells, and this arrangement 

 tends to spread visceral responses rather 

 diffusely, making them less precisely local- 

 ised than somatic reactions. In the autonomic 



