FUNCTIONS OF THE SYMPATHETIC SYSTEM. 439 



centres, sometimes as the result of impressions received 

 through the nerves of general sensibility, produces contrac- 

 tion of the non-striated muscular fibres of the iris, 1 vasa 

 deferentia, etc., including the muscular walls of the blood- 

 vessels. The contraction of the muscular walls of the ves- 

 sels is tonic ; and when their nerves are divided, relaxation 

 takes place, and the vessels are dilated by the pressure of 

 blood. By this action, the local circulations are regulated 

 in accordance with impressions made on sensory nerves, 

 the physiological requirements of certain parts, mental emo- 

 tions, etc. Secretion, the peristaltic movements of the ali- 

 mentary canal, the movements of the iris, etc., are influenced 

 in this way. This action is also illustrated in cases of reflex 

 paralysis, in inflammations as the result of " taking cold," 

 and in many pathological conditions, of which it is not our 

 province to treat. The facts already noted with regard to the 

 excito-motor action of the spinal cord in the functions of ani- 

 mal life have their analogy in the vaso-motor reflex system. 

 When the centres are destroyed, when the sensory nerves 

 are paralyzed by anaesthetics, or when the true vaso-motor 

 nerves are divided, reflex vaso-motor action is abolished. 



The vaso-motor filaments are not confined to the branches 

 of the sympathetic, but they exist as well in the ordinary 

 cerebro-spinal nerves. Bernard has demonstrated this fact 

 in the most conclusive manner. He divided the fourth, 

 fifth, sixth, seventh, and eighth pairs of lumbar nj/ves on 

 one side in a dog, at the spinal column, and paralyzed mo- 



the cerebellum, and that no effects upon the blood-vessels following irritation 

 of the sensory nerves are observed when the encephalon is extirpated, leaving 

 the medulla and cerebellum, or when the sensory nerves are paralyzed by anaes- 

 thetics, Heidenhain presents positive results in opposition to the negative obser- 

 vations of Cycn, at least as far as the experiments after removal of the superior 

 parts of the encephalon are concerned. (HEIDENHAIN, Ueber Cyorfs n.ue Theorle 

 raleti Innervation der Gefiissntrven. Archiv fur die gesammte Physiologic, 

 Bonn, 1871, Bd. iv., S. 551, et seq.) 



1 We assume that dilatation of the iris is produced by the contraction of 

 radiating fibres. Their existence, however, is denied by some anatomists. We 

 will discuss this question fully under the head of vision. 



