110 COMPARATIVE ZOOLOGY. 
Blood-current generally increases with the activity of 
the animal, being most rapid in Birds.” In Insects, how- 
ever, it is comparatively slow; but this is because the air 
is taken to the blood—the whole body being bathed in air, 
so that the blood has no need to hasten to a special organ. 
Nevertheless, the pulsations in a Bee at rest are nearly 
doubled when it is lively. The motion in the arteries is 
two or three times faster than in the veins, but diminishes 
as the distance from the heart increases. In the aorta of 
the Horse, the blood moves 123 inches per second; in that 
of Man, 103; in the capillaries of Man, 2 inches per min- 
ute ; 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 - 
equivalent to lifting 124 tons one foot. 
CHAPTER XIV. 
HOW ANIMALS BREATHE. 
Arterial Blood, in passing through the system, both 
loses and gains certain substances. Its loss is made good 
by fresh products of digestion; and the solid or fluid waste 
matters which it has taken up are removed by excretion. 
