should be involuntary, and so its action is placed beyond our control. It is said that an individual once lived who could stop for an instant, at will, the beating of his heart. But, it is also stated in connection, that he died as the result of a too successful attempt.
The flow of blood through the arteries by successive impulses is facilitated by their branching at acute angles. Veins, on the contrary, branch at greater angles, which is compatible with a steady and slower flow. As the veins carry in any given time the same amount of blood as the arteries, while the rate of flow is slower, it follows that their diameter or capacity is greater.
The pressure of the blood-current diminishes from the heart. In the carotid artery of man it is probably equal to the weight of one hundred and fifty to two hundred cubic millimetres of mercury. The pressure in the pulmonary artery is only thirty to forty cubic millimetres.
There is much disagreement among writers regarding the velocity of the blood. In the carotid artery of the horse, it probably flows at the rate of about three hundred millimetres per second; in the dog, at the rate of three hundred to five hundred millimetres. The velocity in the large arteries of man can hardly be over twenty inches per second, but varies greatly at different times. The length of the capillaries is about one half of a millimetre, and the blood passes through them in about one second. In the human retina the corpuscles travel at the rate of 75 millimetre per second. The small arteries pulsate within one sixth of a second after the main trunks; but the rate of flow is much slower than the wave-progression.
In vertebrates, the rapidity of the circulation is generally proportionate to the activity of the animal. The pulse of aërial birds is about 150 per minute; of the cat, 115; dog, 95; man, 72; ox, 35. But this generalization does not hold with the invertebrates. Insects, the most active of all creatures, have a very sluggish and imperfect circulation, for in this class the air is so freely admitted into the body as to obviate the necessity of great movement of the blood.
The human pulse is somewhat more rapid in childhood, and again in old age; slightly faster in the evening than in the morning, in summer than in winter, and probably increases with geographical altitude. In fever the circulation is very greatly and mysteriously quickened.
All the blood of a man probably completes the round of the circulation in about thirty-two heart-beats, or in less than half a minute. The blood of a horse, it is estimated, completes the circuit in thirty seconds, that of a dog in fifteen, and that of a rabbit in seven seconds.
The velocity of the blood decreases from the ventricles toward the capillaries, and then increases from the capillaries toward the auricles. The velocity being necessarily the reverse of the carrying capacity, or