THE CIRCULATORY SYSTEM 115 



The heart movements may be best studied by stunning a small 

 animal and exposing its beating heart. The series of movements 

 through which the heart goes is termed a cardiac cycle. In the horse 

 at rest it takes about two seconds to complete this cycle. At the 

 beginning there is a contraction of the muscular walls of the heart 

 by which the blood is forced out of the ventricles into the arteries. 

 This movement is called systole. Immediately the heart muscles 

 relax, which causes the auricles to dilate to their full extent and 

 fill with blood from the several large veins emptying into them. 

 This movement is termed diastole. When these movements are 

 completed the heart pauses or comes to a rest, and the cardiac 

 cycle is finished. Although very short, the resting period is suffi- 

 cient to permit the heart to recuperate from its labor so as to be 

 prepared for another similar series of movements. 



An intricate group of nerves control the heart movements. 

 They are derived from branches of the two vagi, each of which has 

 a'plexus near the base of the heart to influence the rate and force 

 of its movements. One set of these nerves conveys impulses which 

 stimulate the heart to increased activity, and are known as the 

 accelerator nerves. Another set of nerve-fibers produce exactly 

 the opposite effect and causes the heart to beat slower and with 

 less force. These are termed inhibitor nerves from their action. 

 In the walls of the blood-vessels are found nerves known as vaso- 

 constrictors and vasodilators. They regulate the size of the vessels 

 by acting upon the muscle-fibers in their walls. When a profuse 

 supply of blood is required by the stomach, immediately after a 

 meal, for example, to furnish the gastric glands which are extremely 

 active at that time with substances to replace those secreted, the 

 vasodilator nerves come into play. The result is a physiologic 

 congestion of blood in the organ. 



The heart-beat is the term commonly used to express the com- 

 plicated series of changes just described. In healthy adult horses 

 the heart beats from twenty-eight to forty times in a minute; in 

 foals the beat is more frequent, varying from forty-five to one 

 hundred times in a minute. 



The heart work is the energy required to pump the blood 

 through the organ. On account of the greater resistance in the 

 peripheral arteries and capillaries than in their pulmonary counter- 

 parts, the work of the left heart has been estimated to be four times 

 that of, the right. The pressure under which blood is forced into 



