228 GENERAL ANATOMY OF THE SANGUIFEROUS SYSTEM. 



thin prolongation of the pulmonary tissue. The impulse of 

 the heart, pulsis cordis, is owing to the shock communicated 

 to the walls of the thorax in the neighborhood of the fifth and 

 sixth ribs, and which is believed to take place during contrac- 

 tion, by the apex of the heart being tilted upwards and for- 

 wards. This impulse must not be confounded with the arterial 

 pulse or proper sounds of the heart, though it is synchronous 

 with the first sound. 



— The sou7ids of the heart are twofold ; the first sound and the 

 stronger, is produced by the quick successive contraction of the 

 auricles and ventricles. The second sound, as results from the 

 recent observations of Dr. Carswell, is produced by the sudden 

 closing under the elastic reaction of the arteries, of the semi- 

 lunar valves of the aorta and pulmonary artery. The interval 

 between these sounds is very slight ; it is calculated by Miiller 

 to be about the fifth part of a second. 



— The pause which intervenes between the termination of the 

 second and the recommencement of the first sound, is nearly equal 

 to the period which elapses between the pulsations of the arteries. 



Of the Pulse. 



— The blood not being able to escape from the arteries into the 

 veins through the capillaries, as quickly as it is injected into 

 the former by the heart, it necessarily exerts a pressure on the 

 elastic coat, which yields both in the direction of its length and 

 its diameter. This elastic yielding constitutes the arterial throb 

 or pulse. This pulsation is not produced, however, directly 

 by the two ounces of blood injected that moment from the 

 heart ; but indirectly, in consequence of these two ounces being 

 injected into the base of the column of blood with which the 

 aortic system is filled so as to transmit the shock through 

 the whole arterial tubes, driving out at the other extremity a 

 corresponding quantity into the veins. The impulse of the 

 heart is transmitted by the oscillation of the particles, along the 

 vessels, (precisely as takes place when a shock is communicated 

 to a column of water in a hose,) and this constitutes the pulse. 

 The elastic force of the arteries when distended is very familiar 



