592 THE SYMPATHETIC SYSTEM. 



following them to their minutest ramifications. They appear to act by 

 causing a contraction in the organic muscular fibres of the small arteries, 

 thus regulating the resistance of the vessels, and the' passage of the 

 blood through them. When the sympathetic nerve is excited, the vessels 

 contract, the blood passes through them slowly, and is fully converted, 

 during its passage, into venous blood. When the influence of this nerve 

 is diminished or suspended, the vessels dilate, and the blood, passing 

 through them with greater rapidity, is not completely changed from the 

 arterial to- the venous condition. 



Connection with Reflex Actions. The influence of the sympathetic 

 nerve upon the thoracic and abdominal viscera has been only imperfectly 

 investigated. It undoubtedly serves as a medium of reflex action between 

 the sensitive and motor portions of the digestive, excretory, and gene- 

 rative apparatus ; and it is certain that it takes part in reflex actions 

 in which the cerebro-spinal system is also interested. There are accord- 

 ingly three different kinds of reflex action, taking place wholly or par- 

 tially through the sympathetic system, which may occur in the living 

 body. 



1. Reflex actions taking place from the internal organs, through the 

 sympathetic and cerebro-spinal systems, to the voluntaiy muscles and 

 sensitive surfaces. The convulsions of children are often due to the 

 irritation of undigested food in the intestinal canal. Attacks of indi- 

 gestion may also produce temporary amaurosis, double vision, stra- 

 bismus, and even hemiplegia. Nausea, and a diminished or capricious 

 appetite, are prominent symptoms of early pregnancy, induced by the 

 condition of the uterine mucous membrane. 



2. Reflex actions taking place from the sensitive surfaces, through 

 the cerebro-spinal and sympathetic systems, to the involuntary muscles 

 and secreting organs. Exposure of the integument to cold and wet is 

 often a determining cause of diarrhoea. Mental and moral impressions, 

 excited through the special senses, will affect the motions of the heart, 

 and disturb the acts of digestion and secretion. Terror, or an absorb- 

 ing interest of any kind, will produce dilatation of the pupil, and com- 

 municate in this way an unusual expression to the eye. Disagreeable 

 sights or odors, or even unpleasant occurrences, are capable of hastening 

 or arresting the menstrual discharge, or of inducing premature delivery. 



3. Reflex actions taking place, through the sympathetic system, from 

 one part of the internal organs to another. The contact of food with 

 the mucous membrane of the intestine excites a peristaltic movement in 

 its muscular coat. The mutual influence of the digestive, urinarj', and 

 internal generative organs upon each other is exerted through the 

 medium of the sympathetic ganglia and nerves. The variations of the 

 capillary circulation in different abdominal viscera, corresponding with 

 the activity or repose of their associated organs, are due to a similar 

 nervous influence. These phenomena are not accompanied by conscious 

 sensation, nor by any apparent intervention of the cerebro-spinal system. 



