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 12£ 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 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 before 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 XIY. 
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 
2 *ains 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 
