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One of the vertebrates that arouses the greatest interest among large and small are birds, with more than ten thousand species they constitute the most diverse and successful vertebrate group.

Its flight has fascinated humans for millennia. Greek mythology already puts us on the track of its magnetism with the legend of Icarus, the young man who made wax wings and who, despite the advice of his father, flew so high and so close to the sun that the wings were melted and ended up dying after falling into the void.

Flying is much more than a physical exercise, it is an activity that carries a high energy cost and for which millions of years of evolution have been necessary. The origin of birds must be sought in theropods. The oldest fossil in this group is the Archaeopteryx, which lived in the Upper Jurassic about one hundred and fifty million years ago.

Wing-shaped modified limbs

All animals that fly or have flown throughout history have done so thanks to wings. Although it is true that those of birds, insects, pterosaurs and bats are structurally different, and obey different evolutionary strategies.

Basically it can be said that the wings of birds are nothing more than a modified forelimb, consisting of the same bones as a vertebrate: humerus, ulna and radius. However, their fingers and metacarpals have been fused, they only retain the tips of three original fingers and a braided phalanx.

The wings are aerodynamically designed and covered with feathers, which can be of various types: primary t-shirts (they are inserted in the bones of the “hand”), secondary t-shirts (they are inserted on the ulna), tertiary t-shirts (they are shorter and less numerous, they are inserted in the humerus) and, finally, the covert feathers, which complete the coating.

Powerful pectoral muscles

In addition to all these feathers are the wheelhouses, located in the “tail” and whose function is to modify the direction of flight. Actually, the tail of birds is a bone structure that has originated from the fusion of the caudal vertebrae (pygostyle).

To move the wings the existence of a powerful chest muscles, which is fixed, on the one hand, to the keel – a structure located in the sternum – and, on the other, to the humerus, from where it makes the traction.

The pectoral muscle achieves the necessary momentum for take off, which would be equivalent to the descending phase of the wing; For the next movement – flapping the wing up – it is necessary to involve the supracoracoid muscle, which is located immediately below the pectoralis.

To understand the important role that these two muscles play, go ahead a data, represent approximately one fifth of body weight.

The prominence of the muscles requires a significant supply of oxygen. For this reason, they have a respiratory system that maintains a continuous flow, in such a way that with inspiration the air does not go directly to the lungs, but rather it is stored in bags —air bags— from which a continuous flow is generated.

Once the blood has been oxygenated, the air is projected to other air sacs, where it is stored until the moment of expiration arrives.

Releasing ballast

Having seen the ailerons and engines, and following the simile of airplanes, then we would have to analyze its fuselage. It is made up of dorsal vertebrae – welded together – and by a sacrum fused with the pelvis, in such a way that a consistent and resistant cylindrical structure has been created, which is capable of withstanding the stresses generated during flight.

Nature has eliminated all those anatomical structures that are useless and heavy. Thus, for example, birds lack teeth, which would require strong jaws and powerful and heavy muscles to move them. For the same reason, they also do not have a bladder – except for ostrich and rhea – so urine flows directly from the kidneys to the cloaca.

M. Jara
M. Jara

Pedro Gargantilla is an internist at the Hospital de El Escorial (Madrid) and the author of several popular books.

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