504 THE NERVOUS SYSTEM. 



The sympathetic nerves accordingly exert an influence on the muscu- 

 lar coat of the arteries similar to that of the cerebro-spinal nerves on 

 the voluntary muscles. They cause contraction of these vessels, a dimin- 

 ished flow of blood through them, and consequently pallor and coolness 

 in the corresponding parts. On the other hand, division of these 

 nerves causes relaxation of the arteries and all the secondary results of 

 an increased supply of blood. 



Centres of Origin of the Vasomotor Nerves. From facts above 

 detailed it is evident that the vasomotor nerves of the head and face 

 come from below. They ascend in the cervical portion of the sympa- 

 thetic nerve, and pass, through the superior cervical ganglion, to their 

 distribution in the blood-vessels. The superior cervical ganglion is 

 itself, in some degree, a source of power for these nerves ; and its extir- 

 pation produces complete and durable vascular relaxation in the parts 

 above. But it receives at least a portion of this power from the 

 sympathetic nerve in the neck ; since division of the nerve below the 

 ganglion is sufficient to cause a distinct congestion in the corresponding 

 parts. 



The real origin of the vasomotor fibres of the sympathetic is in the 

 spinal cord. All the sympathetic ganglia, beside their connection with 

 each other by the longitudinal filament of the sympathetic nerve, are 

 connected with the adjacent spinal nerves by communicating branches ; 

 and many of the fibres composing these branches may be traced, 

 through the spinal nerve roots, to the spinal cord. Furthermore, 

 experiment also shows that the spinal cord is the source of nervous 

 action for the sympathetic system. 



This was first proved by Budge and Waller* in regard to the action 

 of the sympathetic on the radiating fibres of the iris. They found in 

 the rabbit a region in the spinal cord, extending from the first cer- 

 vical to the sixth dorsal vertebra, within which galvanization pro- 

 duces dilatation of the pupil, as if the sympathetic itself had been gal- 

 vanized ; but if the sympathetic be previously divided in the neck 

 on one side, galvanization of the cord is without effect on the pupil 

 of the corresponding eye, while it still causes dilatation on the side 

 where the sympathetic is entire. The stimulus therefore passes, in 

 this instance, through the spinal nerve roots and their branches to the 

 ganglia at the root of the neck, and thence upward, through the cervical 

 portion of the sympathetic to the head. The part of the spinal cord 

 where galvanization produces its maximum effect on the pupil is that 

 included between the second and third dorsal vertebra. 



It was subsequently shown by Bernard f that the vasomotor fibres 

 for the head emanate from the spinal cord in the same region, but 

 at a slightly different level ; so that the fibres going to the iris and 

 those influencing the blood-vessels are distinct though adjacent in their 

 origin from the cord. If, in the dog, the roots of the first two dorsal 

 nerves be divided within the spinal canal, all the phenomena connected 



* Comptes Kendus de 1' Academic des Sciences. Paris, 1851, tome xxxiii., p. 372. 

 f Ibid., 1862, tome lv., p. 383. 



