Regeneration of Limbs
Most people would agree that it would be wonderful if humans could regenerate limbs. Those who have lost their arms or legs would be complete again. The day is still far off when this might happen. But in the last 10 years, doctors have reported regeneration in smaller parts of the body, most often fingers.
Regeneration is not a newly-discovered process. For centuries, scientists have seen it work in some kinds of animals. Break off a lizard's (蜥蜴的) tail, for example, and it will grow a new tail. Scientists now are looking for a way to turn on this exciting ability in more highly-developed animals, including humans. Their experiments show that nerves, cell chemistry and the natural electric currents in the body all seem to have a part in this process.
The body of every animal contains general purpose cells that change into whatever kind of cells the body needs. Animals such as the lizard or salamander (蝾螈) use these cells to regenerate a new tail or leg when the old one is broken off. These cells collect around the wound. They form a mass called a blastema (胚基). The cells of the blastema begin to change. Some become bone cells, some muscle cells, some skin cells. Slowly, a new part regrows from the body outward. When completed, the new part is just like the old one.
More than 200 years ago, Italian scientist Luigi Spallanzani showed that younger animals have a greater ability to regenerate lost parts than older animals. So do animals lower on the ladder of evolutionary development. The major difference seems to be that less-developed animals have more nerves in their tails and legs than humans do in their arms and legs.
Another helpful piece of information was discovered in the late 1800s. Scientists found that when a creature is injured, an electrical current flows around the wound. The strength of the current depends on how severe the wound is and on how much nerve tissue is present.
In 1945, American scientist Meryl Rose tested another idea about regeneration. He thought a new limb might grow only from an open wound. Doctor Rose cut off the front legs of some frogs, below the knee. He kept the wounds wet with a strong salty liquid. This prevented skin from growing over the wounds. The results were surprising. Frogs do not regenerate new legs naturally. But these frogs began to grow new limbs. About half of each cut-off leg grew back again. New bones and muscles developed.
This research has led doctors to new ways of treating cut-off fingers. Doctors have observed, for example, that many children and some adults will regrow the top of a finger if the wound is left open.
11 The passage indicates that
A humans can never regenerate limbs.
B humans might be able to regenerate limbs in the future.
C human limbs may be regenerated on some animals first.
D regeneration of human limbs will soon become a reality.
12 Which of the following statements is NOT true？
A Regeneration is a process discovered centuries ago.
B How severe a wound is determines the strength of the current flowing around it.
C The lizard, unlike man, is a less-developed animal.
D Scientists have lately found that a lizard will grow a new tail if the old one is broken off.
13 Regeneration of a part of the body is impossible without
A general purpose cells.
B bone cells.
C muscle cells.
D skin cells.
14 What kind of animal has a greater ability to regenerate a lost part？
A Younger and more highly-developed ones.
B Younger and less-developed ones.
C Older and less-developed ones.
D Older and more highly-developed ones.
15 In Dr. Rose's test, frogs with cut-off legs
A didn't survive.
B began to grow new limbs.
C bled freely from their open wounds.
D started to grow tails.
【参考答案】
11. B 12. D 13. A 14. B 15. B