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 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 



