Action of the Heart. 515 



tide from four to six lines. These different thicknesses indicate the 

 different degrees of strain they are subjected to. The auricles have 

 light work in simply filling the empty and unresisting ventricles. The 

 work of the right ventricle in sending the blood only the comparatively 

 short distance to the lungs is also light ; but that of the left ventricle in 

 driving it to the extremities of the body is much heavier, and it is a cor- 

 respondingly stouter muscle. 



There are nerves and ganglions in the heart which serve to co-ordinate 

 the contractions. In the frog the dominating center is in the auricles, 

 or in the septum between them, and while the heart is in running order, 

 the due ry thm and sequence of contraction is maintained. In man the two 

 auricles contract simultaneously, and their contraction is immediately 

 followed by the simultaneous contractions of the two ventricles. Then 

 there is a slight pause, when the auricles contract again, and so on. 

 From the central ganglia of the heart, fibres ramify to all parts of the 

 heart and are said to connect with all the muscle fibres. If a frog's heart be 

 detached from the body it will continue to contract in due rythmic 

 sequence, but if the auricles be separated from the ventricle, both parts 

 will continue their beatings, but at different rates. Parts of the heart 

 which do not contain ganglia, pulsate as well as those which do. These 

 facts prove that the ganglia only regulate the pulsations. After the 

 heart of a rabbit is detached from the body, it may, under favorable con- 

 ditions, pulsate for 36 minutes, and in one instance the last contraction 

 of the auricle took place 15 hours after death. In a mouse's heart the 

 last pulsation has been observed 46 hours after death, in a dog's 96 

 hours, in a frog's heart 60 hours, in that of a human embryo of three 

 months, four hours. The right auricle is the last part to give up. The 

 heart, like other muscles, is nourished and kept up by food supplied by 

 the blood. It is furnished by two arteries, a right and a left, called the 

 coronary arteries, which leave the aorta near its origin in the left ven- 

 tricle. They send blood into numerous capillaries which ramify through 

 the substance of the heart. The coronary vein conveys the blood, after 

 it has done duty in the heart, back into the right auricle where it is 

 thrown into the general circulation again. When a part of the blood 

 was cut off from the heart of a large dog by the ligature of a large 

 branch of a coronary artery, the ventricles ceased to beat in two and a 

 half minutes, while the auricles continued to pulsate for several minutes 

 longer. After a mammal heart has ceased to beat from exhaustion, it 

 may be started up again by injecting fresh arterial blood into the coro- 

 nary arteries. The left coronary artery of a man became plugged up so 

 as to arrest the flow of blood into the heart, and his pulse fell from 

 80 to 8 beats per minute. ( Landois. ) 



In order to produce the electrical motive power of any muscle, includ- 



