XI THE SYMPATHETIC SYSTEM 495 



supplying the ear and the skin of the head generally, are 

 made to contract, as has been already mentioned, by 

 nervous fibres derived iinmediatel}' from the sympathetic 

 (Fig. 22, C.Sij.). These filires are non-medullated and 

 arise from culls situated in the superior cervical ganglion. 

 The ganglion in turn is connected with the spinal cord by 

 medullated fibres which do notarise from the sympathetic 

 ganglia, but simply pass through them on their way from 

 the upper dorsal region of the cord. Irritation of this 

 region of the cord produces the same effect as irritation of 

 the vaso-motor nerves themselves, and destruction of 

 this part of the cord paralyses them. 



It has, however, been further shown that the nervous 

 influence does not originate here, but proceeds from 

 higher up, from the medulla oblongata in fact, and simply 

 passes down through this part of the spinal cord on its 

 way to join the sympathetic nerves. 



9. The Ssmipathetic Nervous System.— The sym- 

 pathetic system consists chiefly of a double chain of 

 ganglia lying at the sides and in front of the spinal 

 column, and connected with one another, and with the 

 spinal nerves, b^' coumiissural cords (Fig. 142). From 

 these ganglia, nerves are given off which for the most 

 part follow the distribution of tlie blood-vessels, but 

 which, in the tliorax and abdomen, form great networks, 

 <u- plexiises, upon tlie heart and about the stomach and 

 other abdominal viscera. Every efl'erent impulse which 

 passes along the sympathetic system leaves the spinal cord 

 by a medullated fibre — a jjortion of a neuron the cell of 

 which is in the grey matter. This neuron ends in the 

 sympathetic system (either in a ganglion or elsewhere) 

 in connection with the cell of another neuron, non- 

 medullated, which carries the impulse to the end organ. 

 Some of these non-medullated fibres run back into the 

 spinal nerves for distribution to the blood-vessels of the 

 limbs. 



By means of the sympathetic nerves the muscles of the 

 vessels generally, and those of the heart, of the intestines, 



