CHAPTER XXXI 



THE AUTONOMIC NERVOUS SYSTEM 



It has been mentioned that the smooth muscles and glands 

 of the body are innervated by fibres of the general visceral 

 efferent component system. It is characteristic of such fibres 

 that they do not reach all the way from the central nervous 

 system to the effector in question, but they make synaptic 

 connexions with other neurons which carry the impulses on 

 to the muscle or gland as the case may be. There are, there- 

 fore, two members in each efferent circuit of this kind : a 

 connector neuron and an exciter neuron. The cell-body of 

 the exciter neuron may be in a sympathetic ganglion, or it 

 may be by itself near the muscle which it innervates. In the 

 former case, the connector neuron is often called the pre- 

 ganglionic fibre, and the exciter the postganglionic fibre. 

 Impulses conveyed in this way through the visceral efferent 

 system to smooth muscles and glands are involuntary, and the 

 neurons and ganglia concerned in the conduction of these 

 impulses form the autonomic or involuntary nervous system. 

 It may be noticed that the autonomic system is essentially 

 efferent. Although the afferent visceral neurons run up from 

 the viscera through the ramus communicans, and accompany 

 the efferent neurons, they conform to the type of the somatic 

 afferent fibres in that their cell-bodies are in the ganglia on 

 the dorsal roots, and that they stretch all the way from the 

 sense-organ to the central nervous system. After separating 

 off the autonomic nervous system, what is left is called the 

 cerebro-spinal nervous system, including the brain, spinal 

 cord, and the somatic fibre-systems. 



The autonomic nervous system can be separated into two 

 divisions, each of which works against the other. The visceral 



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