THE AUTONOMIC NERVOUS SYSTEM 753 



The sacral autonomic fibres run in the pelvic visceral nerve. The 

 cell-station is in the hypogastric plexus at the base of the bladder. 

 The nerve is vaso-dilator to the penis; motor to bladder, colon, 

 rectum; inhibitory to sphincter of bladder and retractor penis. 



Afferent Fibres. Thus far we have dealt only with the efferent 

 fibres, which form a great preponderance of the fibres of these systems. 

 The afferent fibres make no connections with the cells of the various 

 groups of ganglia, their cells of origin being situated in the posterior 

 root ganglia. Their main function, so far as is known, consists in 

 carrying up impulses which help to regulate blood-pressure. Stimu- 

 lation of the central end of a white ramus causes a rise in arterial 

 pressure, due to increased vaso-constriction ; stimulation of the central 

 end of the afferent nerve from the heart (the depressor) causes a 

 great fall of blood-pressure, due to inhibition of vaso-constriction, 

 especially in the splanchnic area. It is quite possible that the efferent 

 splanchnic fibres to the liver are reflexly affected through afferent 

 fibres of this system; stimulation of the depressor nerve and states 

 of asphyxia bring about a conversion of glycogen to sugar. 



The reflexes of these afferent fibres probably play a great part in 

 maintaining the tone of the abdominal viscera. Normally we are not 

 conscious of our viscera, and they are not sensitive to touch or a 

 cutting instrument. Pain is produced, however, by the distension 

 of any hollow viscera, and under certain pathological conditions pain 

 may arise; but the pain is referred to those parts of the body wall 

 which are supplied bv the same nerve roots as the diseased viscera 

 (Fig. 387, p. 681). 



