THE VASOMOTOR NERVES. 191 



The vasomotor nerves to the kidney and the abdominal viscera 

 are for the most part supplied by the lower thoracic nerves. 

 These sympathetic fibers are combined and enter the abdomen in 

 what are known as the splanchnic nerves, which terminate about 

 nerve cells in a ganglion behind the stomach, which is called the 

 st milnnar ganglion of the solar plexus. If while the normal vol- 

 ume of the kidney is being recorded, the splanchnic nerve of the 

 corresponding side of the body is cut, the kidney will show in- 

 crease in volume, due to the loss of the vasoconstrictor nerve 

 control on its vessels. On the other hand, stimulation of the cut 

 end of the splanchnic nerve leading towards the kidney, will 

 produce a great decrease in the kidney volume, and a marked 

 increase in the systemic blood pressure, due to a diminution in 

 the volume of the vessels of the kidney and of the whole splanch- 

 nic area, since the splanchnic nerves supply not only the kid- 

 neys, but the whole intestinal tract with vasoconstrictor fibers. 



Vasodilator Nerves. There is another class of efferent nerve 

 fibers to the arteries, which are known as the vasodilator nerves. 

 When stimulated they bring about an actual dilation of the arte- 

 rioles, and allow a greater amount of blood to pass through the 

 vessels. Vasodilator nerve fibers are found in all the spinal 

 nerves, and they run to the blood vessels along with the nerve 

 trunks supplying the various organs. Unlike the vasoconstrictor 

 nerves, they do not seem to be continually exerting an influence 

 or tonic action on the blood vessels. Because their action 

 is hard to elicit, not so much is known of their normal 

 functions as is known of the vasoconstrictor nerves. In some 

 nerves, however, they predominate and their action is easily 

 seen. Such is the case in the chorda tympani, a nerve coming 

 from the seventh cranial nerve and supplying the submaxillary 

 gland with fibers, which when stimulated bring about an increase 

 in the flow of saliva and marked dilation of the blood vessels of 

 the gland (see p. 41). Since the dilator fibers offer more resist- 

 ance to cold, and live longer after they are severed from their 

 nerve cells than vasoconstrictor, it is possible to demonstrate the 

 presence of these fibers in the nerve trunks of the arms and legs. 

 It is only necessary to cut the nerves to the extremity a few days 



