CIRCULATION. 465 



lie in the blood-vessels. Ludwig and Cyon knew that the dilatation of 

 the intestinal vessels could produce a great fall in the blood-pressure and 

 turned at once to them. Section of the splanchnic nerve caused a dilata- 

 tion of the abdominal vessels and a fall in the blood-pressure. Stimula- 

 tion of the peripheral end of the cut splanchnic caused the blood-pressure to 

 rise even beyond its former height. If now the depressor lowers the blood- 

 pressure chiefly by affecting the splanchnic nerve reflexly, the stimulation of 

 the central end of the depressor after section of the splanchnic nerves ought to 

 have little effect on the blood-pressure. This proved to be the case. The 

 depressor, therefore, reduces the blood-pressure chiefly by lessening the tonus 

 of the vessels governed by the splanchnic nerve, thus allowing their dilatation 

 and in consequence lessening the peripheral resistance. 



It has already been said that the depressor fibres pass from the heart to the 

 vaso-motor mechanism in the central nervous system. The cardiac fibres are 

 probably stimulated when the heart is overfilled through lack of expulsive 

 force or through excessive venous inflow, and, by reducing the peripheral resist- 

 ance, assist the engorged organ to empty itself. 



The depressor nerve is not in continual action ; it has no tonus ; for the sec- 

 tion of both depressor nerves causes no alteration in the blood-pressure. 



The many successors of Cyon and Ludwig have added relatively few im- 

 portant facts to their extraordinary investigation. 



Sewall and Steiner 1 have obtained in some cases a permanent rise in blood- 

 pressure following section of both depressors, yet they hesitate to say that the 

 depressor exercises a tonic action. 



Spallita and Consiglio 2 have stimulated the depressor before and after the 



FIG. 122. Showing the fall in blood-pressure and the dilatation of peripheral vessels from stimula- 

 tion of the central end of the depressor nerve (Bayliss) : A, curve of blood-pressure in the carotid artery ; 

 B, volume of hind limb, recorded by a plethysmograph ; C, electro-magnet line, in which the elevation 

 shows the time of stimulation of the nerve ; D, atmospheric pressure-line'; E, time in seconds. 



section of the spinal accessory nerve near its junction with the vagus. They 

 find that after section of the spinal accessory, the stimulation of the depressor 

 1 Sewall and Steiner, 1885, p. 168. 2 Spallita and Consiglio, 1892, p. 42. 



30 



