426 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



vision of the right or left splanchnic nerve leads in the course of a 

 few moments to a marked diminution in the pressure. This effect 

 may be rendered even more conspicuous by dividing both nerves. 

 If the distal (abdominal) end of this nerve is now stimulated with a 

 current of moderate strength and duration, it will be noted that the 

 systemic blood pressure rises rather abruptly and remains high for 

 some time after the cessation of the stimulation. All vasomotor reac- 

 tions, however, develop slowly, for the reason that smooth muscle 

 cells do not contract so rapidly as the striped variety. Neither is 

 it possible to continue an experiment of this kind for any considerable 

 length of time, because it is eventually cut short by fatigue. It has 

 been mentioned above that the excitation of the central (thoracic) 



Fig. 225. — Record of the Carotid Blood-pressure in Rabbit During Stimulation 

 OF the Left Greater Splanchnic Nerve. 



end of this nerve with currents of low frequency and strength gives 

 rise to a general vasodilatation and fall in the systemic blood pressure. 

 In explaining this reaction it should be borne in mind that the di- 

 vision of the splanchnic nerve is soon followed by a relaxation of the 

 blood-vessels innervated by it. Consequently, a steadily increasing 

 quantity of blood must leave the systemic channels and become lodged 

 in those of the splanchnic organs. In some animals, this transfer 

 of blood may lead to circulatory disturbances which actually endanger 

 their life. At all events, the fall in general pressure resulting from the 

 engorgement of the splanchnic blood-vessels, eventually gives rise 

 to cerebral anemia and various symptoms, such as vertigo, mental 

 lethargy and muscular weakness. Conditions constantly arise in our 

 system which require extra amounts of blood to be transferred from 

 place to place and especially when the digestive organs are actively 

 engaged in reducing and absorbing the food. This means that they 

 must be supplied with larger quantities of blood which, on being with- 



