HEART STIMULANTS 95 



others decreasing its power. One part of the muscle a 

 band between auricles and ventricles known as the excito- 

 motor area is concerned in cardiac rhythm, and if stimu- 

 lated causes quickening of the heart beat. 



The blood-vessels are regulated in calibre and blood 

 pressure is maintained by the muscle (unstriped) in the arter- 

 iole walls under the control of the vaso-motor centres and 

 nerves. The vessels can be dilated and blood pressure 

 lowered by drugs which either relax involuntary muscle 

 directly or which depress the medullary and spinal centres. 

 On the other hand, stimulation of the vaso-motor centres or 

 of the muscle in the vessel walls causes a constriction and 

 consequent rise of pressure. 



The heart beat may be slowed by stimulating the vagus, 

 either centrally in the medulla, or peripherally in the heart. 

 Thus such agents as strychnine or ammonia, or concentrated 

 chloroform vapour, when first administered, slow the pulse 

 rate of the normal heart by stimulating the medulla, whilst 

 digitalis acts on the peripheral endings to produce the same 

 effect. 



The heart beat may be quickened by agents which either 

 (1) depress the vagus either centrally or peripherally, or (2) 

 stimulate the sympathetic accelerator mechanism. Thus 

 agents like chloroform, chloral, or any of the narcotics by 

 central depression, quicken the heart during the safe stage 

 of anaesthesia or narcosis. Atropine, on the other hand, 

 paralyses the vagal nerve-endings in the heart, and so causes 

 quickening. Again, adrenalin and cocaine afford examples 

 of agents which stimulate the sympathetic and so increase 

 both force and frequency of the beat. 



The heart beat may be strengthened (1) by the last-named 

 group of agents, namely those which stimulate the sympa- 

 thetic accelerator mechanism ; (2) by agents which stimulate 

 cardiac muscle, such as barium, digitalis, and strychnine, 

 and these in small doses slow the beat somewhat. The 

 exeito-motor area is stimulated by larger doses of most of the 

 drugs which stimulate cardiac muscle generally, and so the 

 heart beats become very frequent. There are, in addition, 

 some agents which specially attack this area from the first, 

 notably caffeine and its allies, so that with small doses the 



