听力材料

Task 4

Drill #1 Heart Valve Replacement

People with defective（有缺陷的）heart valves need them replaced, and what’s often used is the heart valve from a pig. It’s called a “porcine valve” because of that: A pig’s valve is very similar to a person’s, and because pig valves are natural and tend to be accepted by the body, patients who receive them require little treatment after surgery…relatively, for transplant（器官移植）patients, I mean. Now, there are problems. For example, pig valves tend to last around ten years---not very long.

Actually, now that I’m on that topic, I might mention that porcine valves are not the only option. Alternative valves have been developed that are entirely artificial.They’re made of plastic and metal. These valves can last for decades---certainly a lot longer than pig valves. But the human body recognizes that plastic and metal are artificial. So what happens is that blood sticks to them, and blood clots（血凝块）form around them. These clots are dangerous because they can block the flow of blood. For that reason, patients who receive artificial valves spend the rest of their lives taking drugs that prevent blood from clotting. The drugs can be tough for the body to

handle, but they’re worth it for the sake of having a functioning heart valve.

Drill #2 Birds of Prey（捕食，掠食）

There are two bird species found in our local area that I think do a good job of exemplifying the range of ways birds of prey can adapt. One is the bald eagle, and the other is the great blue heron（鹭）. They both feed largely on fish, and they’re both large. But the more closely you observe them, the more the differences show.

The bald eagle is always on the lookout（警戒）for dead fish at the side of river or lake or for fish that it can grab from close to the surface of the water. That’s because it has very sharp eyesight and strong talons. It can swoop（俯冲）down, grab a fish in its claws, and start eating it midair,

food into its mouth.

The great blue heron might eat those same fish, but it would have to go about it differently. The heron does have good eyesight----it’d be hard to see prey otherwise, right? But

its claws aren’t as strong as the eagle’s, so it usually attacks by diving into the water head-first. The heron has a long, straight beak that it can use either like a spear（矛，枪，鱼叉）to impale（钉在……上）a fish or like tongs（V形夹子）to grab it.

Drill #3 Behaviorism

One of the earliest behaviorists is Ivan Pavlov, an experimental psychologist noted for his studies of the reflex reaction in humans and animals. A reflex is an involuntary（无意识做出的）action of the body, such as a sneeze, a blink, or a hiccup（打嗝）. Pavlov did a series of experiments using dogs. He found that when a bell is rung each time a dog is fed, the dog starts to associate the sound of the bell with food. Consequently, whenever the bell rings, the dog expects food. The dog salivates（分泌唾液）and drools（流口水）,and there is an increase in the flow of this stomach juices. The dog’s bodily response to the bell is a conditioned reflex, a behavior that occurs because the dog has been trained by the sound of the bell.