616 , NERVOUS SYSTEM 



region of the cord. Impulses generated in these centres, sometimes as 

 the result of impressions received through the nerves of general sensi- 

 bility, produce contraction of the non-striated muscular fibres of the iris, 

 vasa deferentia etc., including the muscular coats of the bloodvessels. 

 The contraction of the muscular walls of the vessels 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 emotions etc. 

 Secretion, the peristaltic movements of the alimentary canal, the move- 

 ments of the iris etc., are influenced in this way. This action also is 

 illustrated in cases of reflex paralysis, in inflammations as the result of 

 " taking cold," and in many other pathological conditions. 



It remains only to show that the phenomena following section of 

 the sympathetic in animals are illustrated in certain cases of disease or 

 injury in the human subject. It is rare to observe traumatic injury 

 confined to the sympathetic in the neck. A single case, however, 

 apparently of this kind, has been reported by Mitchell. A man received 

 a gunshot-wound in the neck. Among the phenomena observed a few 

 weeks after, were contraction of the pupil on the side of the injury, and 

 after exercise, flushing of the face on that side. There was no differ- 

 ence in the temperature of the two sides during repose, but no thermo- 

 metric observations were made when half of the face was flushed by 

 exercise. Bartholow has reported several cases of unilateral sweating 

 of the head (two observed by himself), in some of which there prob- 

 ably was compression of the sympathetic by an aneurism. In the cases 

 in which the condition of the eye was observed, the pupil was found 

 contracted in some and dilated in others. In none of these cases were 

 there any accurate thermometric observations. In a series of observa- 

 tions by Wagner, on the head of a woman, eighteen minutes after 

 decapitation, strong stimulation of the sympathetic produced enlarge- 

 ment of the pupil. In such a case as this, it would not be possible 

 to make observations on the influence of the sympathetic on the tem- 

 perature. 



Vase-inhibitory Nerves. There are certain nerves the direct action 

 of which under faradic stimulation is to dilate certain bloodvessels. 

 These nerves may also be excited by reflex action through the sensory 

 nerves. In many nerves, as the chorda tympani, the nervi erigentes 

 etc., the existence of inhibitory fibres has been demonstrated. For 

 example, division of the nervi erigentes has no immediate effect on the 

 penis, but faradization of the peripheral ends of the nerves dilates the 

 bloodvessels and produces erection. Fibres possessing this property 



