THE BLOOD AND ITS CIRCULATION, 649 



should be involuntaiy, 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 milli- 

 metres. 



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 sec- 

 ond, but varies greatly at different times. The length of the capilla- 

 ries 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 propor- 

 tionate to the activity of the animal. The pulse of aerial 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 circu- 

 lation, 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 alti- 

 tude. In fever the circulation is very greatly and mysteriously quick- 

 ened. 



All the blood of a man probably completes the round of the circu- 

 lation in about thirtv-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 fiftee.i, and that of a rabbit in seven sec- 

 onds. 



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 



