72 THE STRUCTURE AND LIFE OF BIRDS chap. 



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. 



TJie 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, i.e. 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 



