182 The Bird 
in birds. Here we find an organ remarkably large in 
proportion to the size of the bird’s body—a conical knot 
of muscle, the power of which is almost beyond belief. 
The heart of a bird is said to beat a “hundred and twenty 
times a minute when the bird is at rest. The first flap 
of the wings doubles the pulsations, and when the bird 
is frightened or exhausted the number of beats are too 
many to be counted.” 
There are four separate chambers, known as right and 
left ventricles and auricles, and the partition which di- 
vides the heart in the middle is blood-tight so that not a 
particle of “bad” blood can get through and vitiate the 
life-giving stream which has just come from the lungs. 
A Bluebird is perched on a twig near its nest mur- 
muring its sweet warble; a Wood Pewee, half hidden 
in the shadows of some dense, moist forest, speaks to 
us in its sad dreamy phrase; how calmly, how quietly 
they sit! It seems impossible to believe that every drop 
of blood in their bodies is rushing back and forth with 
inconceivable rapidity—from heart to head, from body 
to wings and legs, and back again! 
Let us take the blood as it is just leaving the heart 
in the breast in one of these little feathered beings, and 
trace its course through the body and back again to 
the starting-point. The left ventricle opens into the 
aorta, the greatest artery, or blood-tube leading from 
the heart, in the body. The clean oxygen-food-kearing 
stream rushes through this channel, which we may com- 
pare to the trunk of a tree, and is carried into branch 
arteries, dividing finer and finer, just as the trunk of 
