ACTION OF THE SYMPATHETIC ON THE VESSELS. 309 



is sometimes dependent on the irritation produced by intestinal worms, 

 is explained by supposing that the nutrition of the retina is impaired, 

 owing to the contraction of the vessels, causing a diminution of its 

 supply of blood. It was found by Brown-Sequard, that when one 

 hand was immersed in water at 32, the temperature of the opposite 

 hand fell, though that of the rest of the body remained unaltered ; 

 this effect he attributes also, to the diminution of the nutrient arteries, 

 through the influence of the stimulus upon the vasi motor fibres of the 

 sympathetic system, transmitted to the opposite, but corresponding, 

 part of the body. Whether this effect is due merely to the diminished 

 supply of blood, or partly to the resulting interference with the ordinary 

 nutritive changes of oxidation, is uncertain. Ice, applied to one wing 

 of a bat, causes, in like manner, contraction, or even closure, of the 

 vessels of the corresponding point of the opposite wing. If a freezing 

 mixture be applied to the ulnar nerve at the elbow joint in the living 

 body, the two inner fingers, at first, become slightly colder, but their 

 temperature slowly rises, till they are at length some 9 or 10 warmer 

 than the three outer fingers, owing to paralysis of their vasi-motor 

 nerves ; the temperature of the three outer fingers is probably lowered, 

 on account of the diminished quantity of blood in the radial artery, so 

 that this, in part, accounts for the difference in temperature. Pressure 

 of the finger behind the ramus of the jaw, produces phenomena, some 

 of which are similar to those observed on irritating the sympathetic 

 in the neck ; others, however, are due to irritation of the prieumogas- 

 tric nerves; e. g., heat and tingling of the ear, difficulty of breathing, 

 cardiac and gastric disturbance, and even dilatation, and subsequent 

 contraction, of the pupil. 



The real nature of the dilatation of the vessels, which ensues on di- 

 vision or paralysis of the sympathetic nerves, is uncertain ; some 

 maintain that it is active, and that just as the pneumogastric nerves 

 serve to inhibit, regulate, or restrain the movements of the heart, so, 

 in this case, some active dilating influence is the cause of the relaxa- 

 tion of the muscular coats of the vessels; according to others, how- 

 ever, the dilatation is passive. 



It is remarkable, that movements similar to those already mentioned 

 in the heart, the intestines, and in the coats of the small vessels, may 

 be produced by irritation or division of certain portions of the spinal 

 cord ; those, in fact, with which the sympathetic nerves, supplying 

 any given part, are connected ; so that the influence of the sympa- 

 thetic nerves on the movements of the heart, intestines, and coats of 

 the arteries, would seem to be derived, in part at least, from the spinal 

 cord and medulla oblongata, which are therefore visceri-motor and 

 vasi-motor centres. This appears to be especially true of the heart 

 and stomach. It is even supposed that the constant influence exerted 

 by the sympathetic upon the smaller arteries, is owing to a stimulus 

 conducted to those nerves, but originating in the cerebro-spinal axis. 

 The same is said to be true in regard to its power over the visceral 

 movements. Thus the lenticular ganglion of the orbit has been ex- 

 perimentally shown to be connected with the spinal cord between the 

 sixth cervical and second dorsal vertebra, and also with the back of 



