472 VETERINARY PHYSIOLOGY 



r. FAINTING. 



This is a sudden loss of consciousness produced by failure 

 in the supply of blood to the brain. It is accompanied by 

 loss of control over the muscles. It may be induced by 

 any sudden lowering of the arterial blood pressure, whether 

 due to decreased inflow of blood or to decreased peripheral 

 resistance. 



1. Decreased inflow may be caused by — (1) Cardiac 

 inhibition brought about reflexly {a) by strong stimulation of 

 ingoing nerves, and more especially of the nerves of the 

 abdomen ; (6) by strong stimulation of the upper brain 

 neurons accompanied by changes in the consciousness of the 

 nature of emotions — (2) Failure of the heart to pump blood 

 from veins to arteries against the force of gravity, as when a 

 hutch rabbit is held in the " head up " position for some time. 



2. Decreased resistance to outflow through sudden 

 dilatation of arterioles may result from changes in the 

 upper brain neurons, sometimes as a result of digestive dis- 

 turbances. 



However induced, the anaemic state of the brain leads to 

 a stimulation of the cardio-inhibitory centre and the 

 condition is thus accentuated. In man the cerebral anaemia 

 is accompanied by pallor of the face. 



The treatment consists in depressing the head to allow 

 the force of gravity to act in filling the cerebral vessels and 

 in giving diffusible stimulants to increase the action of 

 the heart. 



G. THE TIME TAKEN BY THE CIRCULATION. 



This has been determined by injecting ferrocyanide of 

 potassium into the proximal end of a cut vein, and finding 

 hoAV long it took to appear in the blood flowing from the 

 distal end. From observation in the horse, dog, and rabbit, 

 it appears that the time corresponds to about twenty-seven 

 beats of the heart, so that in man it should amount to about 

 twenty-three seconds. 



