CHAPTER XVII 



THE AUTONOMIC NERVOUS SYSTEM (THE SYMPATHETIC 

 AND ALLIED NERVES) 



THE efferent fibres of the body can be divided into two classes: 

 (i) Those which supply multinuclear striated muscle (skeletal 

 muscle) ; (2) those which supply other structures (smooth muscle, 

 heart muscle, glands). The second group is called ' autonomic,' 

 to indicate that it possesses a certain independence of the central 

 nervous system, although this independence is far from absolute. 

 The autonomic fibres arise from four regions of the central nervous 

 system : (i) The mid-brain; (2) the bulb; (3) the thoracic and upper 

 lumbar cord; (4) the sacral portion of the cord. All autonomic 

 fibres after issuing from the central nervous system end sooner or 

 later by forming synapses around nerve-cells of sympathetic type, 

 by whose axons the path is continued to the peripheral distribution. 

 The autonomic path accordingly comprises two neurons, the fibre 

 which arises from the brain or cord being termed the ' pregang- 

 lionic,' and that which arises from the sympathetic ganglion the 

 ' postganglionic ' fibre. 



The autonomic fibres originating in the mid-brain emerge in the 

 oculo-motor nerve, and form synapses with cells in the ciliary 

 ganglion, which in turn send fibres to the ciliary muscle and the 

 constrictor muscle of the iris (pp. 923, 1024), The bulbar autonomic 

 fibres emerge in the seventh, ninth, and tenth cranial nerves. Those 

 in the vagus include inhibitory fibres for the heart muscle, motor and 

 inhibitory fibres for the smooth muscle of the alimentary canal from 

 the oesophagus to the descending colon, and for the muscles of the 

 trachea and lungs, and secretory fibres for the gastric glands and 

 the pancreas. The sympathetic ganglion cells with which these 

 preganglionic fibres form synapses have not always been definitely 

 located, but lie near or in the tissue supplied (p. 181). The auto- 

 nomic fibres in the seventh and ninth nerves supply the mucous 

 membranes of the mouth and nose with vaso-dilator and secretory 

 fibres. The preganglionic portion of the path terminates in such 

 ganglia as the submaxillary and sublingual (p. 391) and the spheno- 



palatine and otic ganglia. 



1003 



