24 BRITISH BIRDS 



extent upon shellfish, which the roughened edges of the beak are 

 well suited to crush. The replacement in the course of ages of true 

 teeth by horny teeth is seen a curiously parallel case in the Duck- 

 billed Platypus of Australia, which has when adult horny plates 

 instead of teeth, but when young has real teeth. 



Heart. 



As with all vertebrated animals, birds have a centrally placed 

 heart, with which are connected arteries and veins, the two systems 

 of tubes being connected at the ends farthest away from the heart 

 by minute vessels the capillaries. In relation, no doubt, to the 

 intelligence and activity of birds, as compared with their slower 

 relatives, the reptiles, we find a heart of much more perfect organi- 

 sation. There are four distinct chambers, as in the mammal, so 

 that the arterial and venous blood are separate, and do not com- 

 mingle. The two sides of the heart are only in indirect communi- 

 cation by way of the arteries and veins and capillaries. The left 

 ventricle gives rise to the aorta, which is the great arterial trunk of 

 the heart ; this divides into the carotid and other arteries, which 

 supply the entire body, with the exception of the lungs. The blood, 

 which is sent out through this vessel by the contractions of the ven- 

 tricle, permeates the system generally, and is then collected into a 

 series of veins, which ultimately unite into two great veins, he 

 venae cavae in front, and a large vein situated posteriorly, the in- 

 ferior vena cava. These pour the blood back into the right auricle, 

 whence it passes at once to the right ventricle. From the right 

 ventricle it is driven into the lungs, whence it is returned to the left 

 auricle, and so into the left ventricle to renew the circulation. The 

 two chambers of each half of the heart are guarded from each other 

 by valves, which only allow the blood to flow in the proper direction, 

 as stated in the above brief description of the course of the circulation. 

 It is a curious fact that the valve which separates the right auricle 

 and ventricle is a completely muscular structure, while the other is 

 membranous. Moreover, it does not form a complete circle, but is 

 deficient upon one side of the orifice. The interest of this fact is 

 not merely in its abnormality, its divergence from what one would 

 expect, but in the resemblance which is thus shown to a group of 

 mammals, the Monotremata. This group includes only the Duck- 

 billed Platypus of Australia and the spiny Anteater (Echidna) of the 



