HOW ANIMALS BREATHE. 
Ill 
body being bathed in air, so that the blood has no need 
to hasten to a special organ. However, activity nearly 
doubles the rate of pulsation in a Bee. The motion in 
the arteries is several times faster than in the veins, but 
diminishes as the distance from the heart increases. In 
the carotid of the Horse, the blood moves 12J inches per 
second; in that of Man, 16; in the capillaries of Man, 1 
to 2 inches per minute; in those of a Frog, 1. 
The Cause of the Blood-current may be cilia, or the 
contractions of the body, or pulsating tubes or hearts. In 
the higher animals, the impulse of the heart is not the sole 
means: it is aided by the contractions of the elastic walls 
of the arteries themselves, the movements of the chest in 
respiration, and the attraction of the tissues for the arterial 
blood in the capillaries. In the Chick, the blood moves be¬ 
fore the heart begins to beat; and if the heart of an animal 
be suddenly taken out, the motion in the capillaries will 
continue as before. It has been estimated that the force 
which the human heart expends in twenty-four hours is 
about equivalent to lifting 217 tons one foot. 
CHAPTER XIV* 
HOW ANIMALS BREATHE. 
Arterial Blood, in passing through the system, both 
loses and gains certain substances. It loses constructive 
material and oxygen to the tissues. These losses are made 
good from the digestive tract and breathing organ. It 
gains also certain waste materials from the tissues, which 
must be got rid of. Of these waste products, one, carbon 
dioxide, is gaseous, and is passed off from the same organ 
as that where the oxygen is taken in. This exchange of 
* See Appendix. 
