72, THE STRUCTURE AND LIFE OF BIRDS cuap, 
right and left sides of the head, in man into that from 
the left side only. 
The great difference (its significance will be made 
clear in chap. viii.) between the circulatory systems of 
birds and mammals is this—in mammals the aorta 
arches over to the left, in birds to the right. 
The Valves of the Heart. 
When the black blood is discharged into the heart, 
it has to be sent to the lungs to be purified and to be 
recharged with oxygen, and the heart by contracting 
drives it into the pulmonary artery, ze. the artery 
which leads to the lungs. But unless there is some 
means of preventing it, obviously the blood will be 
driven not only to the lungs, but back into the vein 
which has just carried it to the heart. The right 
side of the heart, therefore, is divided into two 
chambers, the passage between them being guarded 
by a valve which allows the blood to pass from the 
upper chamber to the lower, but not from the lower 
to the upper. In the bird this valve is simply a flap 
of muscle which projects into the ventricle, and which 
closes the aperture when it is lifted by a rush of blood 
upwards. In man and other mammals the valve is 
formed of thin membrane instead of muscle, and 
consists of three flaps connected with one another and 
fastened by strings of tendon to the walls of the’ heart 
below. This is called the tricuspid valve. It is very 
curious that the bird’s heart should be in most respects 
so similar to that of man, in this respect so different. 
These valves remain to show that the two highest 
