184 The Bird 
the tree merges into limbs, and these into branches, twigs, 
stems, and at last into the delicate foliage. This last 
we may liken to the capillaries or hair-tubes in which 
the blood does its real work of supplying nourishment 
directly to the tissues, and where it receives the waste 
matters, carrying them away in its current. 
When we have followed the divisions of a tree out 
to the foliage, we may find that they touch and interlace 
with the foliage of another tree, and this is very much 
like what occurs in the course of the blood. The capil- 
laries run together and form larger vessels, these in turn 
coalesce, and soon the blood—dark now and filled with 
the waste matters of the body-cells—is flowing through 
only two large veins (veins always lead toward the heart). 
These enter the right auricle, which opens into the right 
ventricle. From here the blood rushes to the lungs to be 
purified and back again to the left auricle and ventricle, 
and its cycle is complete. 
If we look at a drop of bird’s blood (or that of any kind 
of warm-blooded creature) under the microscope, we shall 
see thousands upon thousands of oval discs, or corpuscles, 
like tiny platters floating in a fluid. These flow about 
under the cover-glass through little channels, mechanic- 
ally and very slowly of course, and giving but a faint 
idea of the way they must tumble and rush after each 
other through the veins and arteries of the bird. Scat- 
tered among these oval bodies will occasionally be seen 
others of indefinite shape and white in colour. As we 
watch one of these tiny cells, the thought suddenly comes 
over us,—what are birds indeed but collections of untold 
