204 THE AUTONOMIC NERVOUS SYSTEM [CH. XVII. 



hypogastric nerve. The experiment is in fact similar to Kiihne's 

 gracilis experiment (p. 160). They also observed an apparent reflex 

 excitation of certain nerves supplying the erector muscles of the 

 hairs (pilo-motor nerves) through other sympathetic ganglia; but 

 this is explicable in the same way. 



It certainly is the case that under normal circumstances the 

 centres for reflex action are in the central nervous system. But 

 there do appear to be some conditions in which it is possible for 

 ganglia to assume this function. The recovery of vaso-motor tone, 

 and of tone in certain viscera after destruction of extensive tracts of 

 the spinal cord, or the persistence of peristaltic action in the intes- 

 tine after cutting through all its nerves, are cases in point. (See 

 further, under Intestinal Movements, and Spinal Visceral Eeflexes.) 

 Such action forms, in fact, the chief justification for the adoption of 

 the new term, autonomic. 



Afferent Nerves of the Autonomic System. 



Up to the present point, we have only considered the efferent 

 fibres of the autonomic nerves. No survey of the autonomic system 

 will, however, be complete which does not include an account of the 

 afferent fibres. This will not occupy much space, because our infor- 

 mation on this side of the subject is so scanty. 



The " vegetative" functions of the body are carried out inde- 

 pendently of volition, and under normal circumstances they also 

 cause no sensations. In pre-ansesthetic days, surgeons discovered 

 that the viscera possess no sensibility in the ordinary sense ; they 

 may be handled and cut without producing pain ; and, with the 

 exception of the oesophagus, they are insensitive also to heat and 

 cold. 



Still, under abnormal conditions we become conscious of their 

 activity, especially if it is excessive, as for instance in the very severe 

 pain which the various forms of colic give rise to. But even under 

 these circumstances there is great difficulty in accurately localising 

 the pain. 



The afferent or sensory fibres are much less numerous than those 

 which are efferent. This has been ascertained by cutting the 

 anterior nerve roots which communicate with an autonomic nerve ; 

 the efferent fibres will degenerate peripherally, but the sensory fibres 

 will remain intact, and the relative number of healthy and degene- 

 rated fibres can then be counted. Thus in the splanchnic and 

 hypogastric nerves about one-tenth of the fibres are found to be 

 sensory; and in the pelvic nerve about one-third of the total fibres 

 are sensory. 



The grey rami contain few if any sensory fibres; excitation of 



