132 PRINCIPLES OP VETERINARY SCIENCE 



heart of a turtle or frog and placing it in a beaker containing a' 

 balanced salt solution. 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 begin- 

 ning 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. It causes the heart to shorten 

 and become globular. 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 sufficient to permit the heart to recuperate from its labor, so as 

 to be prepared for another similar series of movements. The 

 right and left sides of the heart move synchronously. 



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 the heart's movements. One set of these nerves con- 

 veys impulses which stimulate the heart to increased activity, 

 and are known as the accelerator nerves. Another set of nerve 

 fibers produces exactly the opposite effect and causes the heart 

 to beat slower and with less force, acting as a governor does on 

 an engine. These are termed inhibitor nerves from their action. 

 When the connections between the nervous system and heart are 

 severed the result is a quickening of the beat. This indicates 

 that the prevailing influence of the centers is inhibitory. In 

 the walls of the blood-vessels are found vasoconstrictor nerves 

 and vasodilator nerves, which 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 vasomotor 

 nerve center is located in the medulla. 



The heart beat is the term commonly used for the complicated 

 series of changes just described. In healthy adult horses the 

 heart beats from 28 to 40 times in a minute; in foals the beat is 

 more frequent, varvjrjsfrom 45to 100 ±imes in a minute. 



