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heart of birds
Значение термина heart of birds в knolik
heart of birds - Heart of birds
heart of birds - 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 organisation. There are four distinct chambers, as in the mammal, so that the arterial and venous blood are separate, and do not commingle. The two sides of the heart are only in indirect communication 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 ventricle, 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 inferior 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 Duckbilled Platypus of Australia and the spiny Anteater (Echidna) of the same continent and New Guinea. In both of these animals the heart valve in question is also largely muscular, and does not entirely encircle the opening from the auricle. These two mammals also, as everyone knows by this time, have the strange habit for a mammal of laying eggs, which is one among some other reasons which once led naturalists to place them in the neighbourhood of birds. The egg-laying, of course, is not distinctive, since reptiles have the same way of bringing forth their young; and as to the heart valve, it is rather to be explained by the fact that both types of animals are low in the scale of their respective groups, and therefore both approach a common ancestral form.
Рядом со словом heart of birds в knolik
| tongue and teeth of birds | В начало буква "" буквосочетание "" | gizzard and alimentary canal of birds |
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