1 82 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-bearing 

 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 



