144 INNERVATION. [CHAP. XXI. 



very vigorous. There is less agreement in the statements of the 

 results obtained by different experimenters as to the sensibility of 

 these nerves, as indeed is very commonly the case, when the ques- 

 tion is respecting sensation ; but the well-known occurrence of pain 

 in parts supplied only by the sympathetic, is alone more conclusive 

 as to the existence of sensitive fibres in that nerve, than the results 

 of any experiment on a brute animal. How exquisite are the suf- 

 of patients labouring under colic, or the passage of a gall- 

 stone, or of a renal calculus ! 



It is plain, then, that the sympathetic nerve contains both motor 

 and sensitive fibres. An appeal to common experience shows us, 

 that the latter cannot be very numerous, as parts supplied by the 

 sympathetic nerve are not, in the healthy state, highly sensitive, 

 and pain is felt in them only under the influence of great irri- 

 tation. And with regard to the motor fibres, it shows that they 

 are not at all, or at most to a very trifling extent, under the influ- 

 ence of the will. It is true that the will may be brought to bear 

 upon muscles supplied by the sympathetic, by directing it simul- 

 taneously upon other distinctly voluntary muscles. This is well 

 illustrated in the case of the iris ; no effort, however great, if directed 

 solely upon the iris, will cause that muscle to contract, but if the 

 voluntary influence cause a simultaneous contraction of the internal 

 rectus muscle of the eye, contraction of the pupil will take place 

 upon each adduction of the eyeball. 



It is highly probable that the increase in force and in fre- 

 quency which takes place in the heart's action, during active exer- 

 cise, is to be explained on the same principle ; and that a strong 

 effort of the will directed to the abdominal muscles, may excite an 

 increased peristaltic action of the intestines. 



Muscles supplied by the sympathetic nerve, although under ordi- 

 nary circumstances referrible to the category of involuntary muscles, 

 must not then be considered as absolutely and entirely removed from 

 the influence of the will. 



A very striking peculiarity, dependent in part probably upon 

 the anatomical arrangements of the sympathetic, consists in the 

 rhythmical nature of the movements of parts which derive their sup- 

 ply of nerves from this source, of which the movements of the heart 

 and the intestinal canal afford good examples. And it is an im- 

 portant feature of these actions that they take place even when the 

 parts are disconnected from the main portion of the sympathetic . 

 system. It is well known that the heart's action will go on for a 

 considerable time after it has been removed from the body ; and 



