268 HUMAN PHYSIOLOGY 



domen. Stimulation of the splanchnic nerves greatly 

 reduces the quantity of blood passing through the 

 vessels of the digestive tract. Cutting the splanchnics 

 permits these vessels to widen according to the principle 

 of Bernard's experiment. 



If we abolish arterial tone in an important field like 

 that served by the splanchnic nerves we shall greatly 

 lower the pressure in the aorta and its branches. There 

 are two reasons for this: first, the widened vessels op- 

 pose less resistance to the onward movement of the 

 blood and, second, the capacity of the abdominal vessels 

 is so much increased that they retain an abnormal 

 quantity of blood, leaving so much less to circulate 

 elsewhere. This has been described as " bleeding into 

 one's own veins" and is a common cause of faintness, 

 for example, in severe indigestion. 



Vasodilator Fibers. The experiment of stimulating 

 a nerve to observe the effect upon the blood flow in its 

 field does not always give the constrictor reaction. 

 We have evidence of the existence of an opposing order 

 of vasomotor fibers through which the dilation of small 

 blood-vessels can be promoted. Such are the vaso- 

 dilator fibers. If, for example, we stimulate the small 

 nerve known as the chorda tympani, an offshoot of 

 the seventh cranial, we may see that the blood-flow 

 through the submaxillary gland instead of being reduced 

 is much increased. The result is explained by the 

 assumption that there are fibers in this nerve which 

 are inhibitory to the contractile elements in the asso- 

 ciated vessels. 



Students often find it hard to visualize the action of 

 vasodilator fibers. It may be admitted that it is not 

 a simple matter. How can stimulation of nerve fibers 

 increase the diameter of arteries? We shall probably 

 do well to emphasize the fact of the internal pressure in 

 this connection. An artery will dilate if the usual re- 

 sistance to this pressure is diminished. The chemical 

 changes which occur in the muscle cells under the in- 



