CHAP. XII.] SYMPATHETIC SENSATIONS. 393 



to sympathise with another. In the instance of pain in the shoul- 

 der in sympathy with irritation of the liver, the hepatic irritation 

 excites a change in some sensitive nerves, which is propagated to the 

 centre, and there affects some of the sentient fibres distributed in the 

 region of the shoulder. The phrenic and the external thoracic 

 nerves are both or either of them, but more especially the former, 

 favourably situated to constitute the excitant of such a sympathetic 

 sensation. The phrenic nerve of the right side is largely distributed 

 upon the peritoneal surface of the diaphragm, and upon the inferior 

 vena cava, and forms many connexions with the hepatic plexus in 

 the substance of the liver. It may therefore readily participate 

 in any irritation of that organ. Now the phrenic nerve is im- 

 planted in the spinal cord on a level with the third or fourth cervical 

 nerves ; and the nerves of the shoulder form their connexion with 

 this central organ about the same level. The origins of these 

 nerves are sufficiently contiguous to each other to warrant the 

 belief that an irritated state of one may be propagated to the 

 other through the vesicular matter of the centre. But it may be 

 inquired why the irritation is limited to the sensitive nerves of the 

 shoulder ; and why movements are not excited by the stimulation 

 of the motor fibres of the phrenic itself, or of other nerves ? The 

 limitation of the irritation to one or two nerves depends on the 

 degree of the stimulus, and the absence of movements is due to 

 the disposition of the phrenic on the surface being unfavourable 

 for the excitation of motions by irritation of its peripheral branches 

 (see page 335) . And the experiment cited from Miiller, in the 

 last paragraph, shews that simple irritation of the hunk of a com- 

 pound nerve in connexion with the centre is not sufficient to pro- 

 duce motion ; which requires probably either a more prolonged and 

 violent irritation of the nerve, or a polar state of the centre in which 

 it is implanted. 



Some of the instances of sympathetic sensations, referred to above, 

 do not admit of an explanation so obvious. The pain over the 

 brow from ice or cold water in the stomach may be referred to irri- 

 tation of the gastric branches of the vagus, communicated in the 

 medulla oblongata to the fifth ; but why the irritation should be 

 limited to the ophthalmic division of the fifth cannot be accounted 

 for in the present state of our knowledge. 



In those sympathetic movements which are of ordinary and nor- 

 mal occurrence, two provisions seem to be secured, namely, a certain 

 peripheral organization of the excitor nerve, and a certain central 

 relation between it and the motor nerve. But in those which are of 



