将一段新鲜的猪小肠翻转过来，使内表面朝外，浸入盛有清水的小烧杯中，认真观察，如图所示，可看到小肠内壁上有许多由黏膜折成的______．若再用放大镜仔细观察，还可发现其上有很多______．
人体消化食物和吸收营养物质的主要器官是小肠，如图是小肠的结构图，可见，小肠内表面有许多环形皱襞，皱襞上有绒毛状的突起，叫做小肠绒毛，这大大增加了小肠的内表面积；小肠绒毛内有毛细血管和毛细淋巴管，绒毛壁、毛细血管壁、毛细淋巴管壁都是由一层上皮细胞构成的，有利于营养物质的吸收．
故答案为：皱襞；小肠绒毛
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| Don't Take the Fun Out of Youth Sports When I joined a private football league a few years ago, the sport meant everything to me. My coach (教练)said that I had lots of potential (潜力), and I became captain of my 1 . That was before all the fun was taken out of 2 . At first, everyone on the team got 3 playing time. Then the team moved up to the top division after winning all its games, and the 4 started. Some parents, who had paid the coach extra so their daughters could have 5 one-on-one training, got angry when she didn't give them more playing time in our 6 . The coach was replaced. The new coach, however, took all the fun out of the game: All we did during practice was 7 . I always wished to God that it would rain so we would not have the 8 . Of course, all teams run drills; they are 9 . But we ran so much that, afterwards, we had trouble 10 . Younger people shouldn't be doing exercises 11 for 18-year-olds. I was very thin 12 I started football, but as a member of this team I wouldn't eat much, because I was afraid of being too 13 8 to run. I feared making mistakes, and the added pressure caused me to make more than my usual 14 . Is all this pressure necessary? I 15 up leaving the football team. Four other girls did the same, and two of them stopped playing football completely. That's 16 , because they had so much potential. They were just burned-out with all the pressure they 17 from the coach or their parents. I continued playing football at school and 18 my love for it. I joined a private team coached by my school coach. When I started playing 19 him, he told me I needed to relax because I looked nervous. After I 20 down, I played better. When you enjoy something, it's a lot easier to do it well. | ||||
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