of fifty years much, more vulnerable than the young man; and vulnerable in precisely the organ most essential to life. It is, in fact, the heart that suffers in case of forced exertion, the consequences of a deficient elasticity of the arteries. Every beating of the heart represents the piston-stroke of a force-pump, and the blood-vessels are the pipes through, which the liquid flows to carry life to the furthest molecules of our body. But these vessels are not inert conductors; they are endowed, in a healthy condition, with an elasticity which permits them to react at each pulse of the heart, swelling under the pressure of the sanguineous wave, and then contracting and returning to the liquid the impulse which they have received from it. The liquid, striking upon the wall of a fully elastic artery, does not suffer at once the arrest which it would suffer on meeting a rigid wall. A billiard-ball, driven against a very elastic cushion, rebounds with nearly as much force as it had when it started. An artery which has lost its elasticity is, as to the column of blood that comes against it, as an ivory ball to a cushion that does not spring. And as the billiard-player must strike more vigorously upon the ball to make it perform its run when the cushions do not spring, so the heart, when the artery has lost its elasticity, must exaggerate its effort at the systole to enable every molecule of blood to traverse the circle of the vessels and return to its point of departure. In short, the less elastic the arteries, the greater the effort the heart has to make to secure equal work. Each heart-beat, then, of a man whose arteries have become old, is the occasion for an excess of labor by the cardiac muscle. The increase in expenditure of force passes unnoticed if the beatings retain their normal slowness, but becomes very sensible when they are quickened. There are some exercises which cause the number of heart-beats to double in a few moments. The resultant fatigue of the organ, which has already been brought to the point of overwork by the continual excess of work it has had to do, is easy to conceive.
The most natural consequence of fatigue of the heart is a momentary diminution of its energy; and when the organ is weakened, the impulse it gives to the blood is no longer sufficient to cause it to traverse as rapidly as it ought the vessels through which it circulates with most difficulty, either on account of their narrowness, or of the mass which is precipitated into them at once. Hence what are called passive congestions of the internal organs, and particularly of the lungs. Congestion of the lungs is a frequent consequence in elderly men of exercises which accelerate to excess the rhythm of the pulse, and is shown by shortness of breath. This, which is more prompt in men habituated to physical exercises, is one of the first symptoms of arterial dete-
