THE MECHANISJM OF THE CIRCULATIOIS 441 



the sympathetic nerve. They have been divided into the accelerating 

 and the restraining or inhibitory nerves. The accelerating nerves leave 

 the spinal cord at the fore part of the dorsal region, and pass to the first 

 dorsal ganglion of the great sympathetic chain, and after a short course 

 are distributed to the heart. Stimulation of these branches causes the 

 heart to beat more frequently. 



The inhibitory or restraining nerves of the heart are derived from the 

 medulla oblongata, and run in the vagus nerve. Their peculiarity is that, 

 instead of causing contraction of the muscle to which they are distributed, 

 they induce relaxation, so that when stimulated they stop the heart in 

 diastole, that is, in a condition of relaxation; whilst when less strongly 

 stimulated they cause it to beat more slowly. The effect is not, however, 

 persistent, for even if the vagal branches continue to be strongly stimu- 

 lated, the heart after a time recommences to beat more quickly and more 

 strongly than before. The administration of belladonna, or of its active 

 principle atropine, prevents the inhibitory effect from manifesting itself 

 when the vagus is stimulated, and a similar action is exerted by curara 

 and by nicotine. On the other hand, muscarin, a poison obtained from 

 a mushroom {Amanita muscaria), seems to stimulate or intensify the 

 inhibitory influence. 



The heart is in unceasing movement day and night. Yet it has, like 

 other muscles, its period of rest; for expenditure of force is only taking 

 place during contraction, which occupies about one-half of the whole cycle 

 of its action. It difters, therefore, from the ordinary muscles that are under 

 the control of the will only in the circumstance that, instead of long spells 

 of greater or less activity occurring alternately with the complete rest of 

 sleep, its periods of work and rest have only short intermissions. The 

 force it exerts is immense. If we estimate that the quantity of blood 

 driven out of the left ventricle at each contraction is the low amount of 

 1 pint or pound, and that it is raised about 10 feet, which represents the 

 blood pressure, the work done is 10 foot-pounds per beat. Taking the 

 number of beats at 40 per minute, we have 400 lbs. raised 1 foot per 

 minute, or 1 lb. raised 400 feet. If this be multiplied by the number 

 of minutes in an hour, and of hours in the day, the surprising number 

 of 576,000 foot-pounds, or more than 257 foot-tons, raised in twenty-four 

 hours is obtained, which represents the work done by the left ventricle. 

 The work of the right ventricle is estimated at one-third of this amount. 

 The duration of one complete circulation of all the blood is about 27 

 seconds. 



The Blood-vessels. — The first vessels into which the blood is driven 

 by the lieat of the heart are the arteries. These are tubes which conduct 



