404 PATHOLOGY AND TllEATMEXT OF SHOCK AND SYNCOPE. 



them, and, as Goltz's experiment shows, the heart may be pul- 

 sating and yet the ciiculation be at a standstill Now, the 

 arterial tension may be lessened (1) by the heart stopping, or 

 (2) sending little blood into the arteries at each beat, or (3) by 

 the arterioles dilating so as to let the blood easily out from the 

 arteries into the veins, where it may stagnate and be useless. 

 Another point we must remember is that it is the circulation in 

 the brain which is the important factor in producing insen- 

 sibility. In the patient who was suddenly lifted on his feet the 

 circulation in the rest of the body was going on perfectly well ; 

 it only failed in the brain. 



Now, it is very easy to bring down the blood-pressure very 

 much by stopping the heart for a few instants, and it may take 

 a little while before it rises to its normal condition. A second 

 w^ay is to dilate the arterioles, and if the arterioles be dilated at 

 the same time that the heart is stopped, the pressure will fall 

 with great rapidity, and, when the heart again begms to beat, it 

 will take a much longer time to raise the pressure sufficiently to 

 ■carry on the circulation than it would otherwise do. Now, 

 when a painful impression is made on a sensory nerve, it is not 

 unfrequently carried up to the medulla oblongata, where it is 

 transferred to the vagus nerve, which, as you know, has the 

 power of stopping the heart, and by setting this nerve into 

 action arrests the cardiac pulsations. If the arterioles should 

 happen to be dilated, as they almost always are in a warm 

 room, the pressure of blood in the arteries immediately sinks, 

 the brain getting an insufficient supply ceases to act, and the 

 patient falls down unconscious. The very fact of the head being 

 lowered induces more blood to joass to it, and the normal con- 

 dition is at once in many cases restored. 



The condition of the vessels in fainting has not been ascer- 

 tained, and the only observation bearing on the subject that I 

 can find is one by John Hunter.* While engaged in bleeding 

 a lady she fainted, and during the continuance of the faint he 

 observed that the blood which flowed from the vein, instead of 

 being dark and venous, was of a bright scarlet colour, like that 

 of arterial blood. Now, the only condition in which we know 



• W'orlcs of John Hunter, edited by Palmer, 1837, Tol. iii, p. 91. 



