【英语时差8,16】头发的生长

 Hair Grows And Grays Whether you have a full head of curly blond hair or a nearly bald head with dark hair, the growth of your hair begins in the skin on your scalp. On the surface of your head, the outer layer of cells–called the epidermis¹–is between ten and twenty layers thick. Some of these epidermal² cells actually become hair cells by a complex process. To visualize³ this process, think about taking some plastic kitchen wrap and covering a tub of cream cheese with it. If you poked⁴ your finger into the wrap, and down into the cream cheese without breaking the wrap, you would be simulating the beginning of hair development. In order for a strand⁵ of hair to grow, skin cells must dive down to just below the epidermis. This creates a tube-like structure, much like the one left over if you pulled your finger out of the cream cheese. The cells that line this tubular structure are collectively referred to as the hair follicle. Hair production is the result of the cells of the hair follicle depositing layer after layer of protein into this tubular space. These layers of protein grow out of the tubular space as a strand of hair. As the hair strand develops, hundreds of pigment⁶-producing cells called melanocytes inject it with pigment, giving a strand of hair its color. Melanocytes are only able to produce pigment for a genetically-determined number of years. Once the melanocytes lose their ability to produce pigment, hair follicles continue to produce hair, but instead of brown or red hair, they produce gray hair, which, technically⁷ speaking, is hair with no color at all.

1 epidermis   

n.表皮

参考例句：

• The external layer of skin is called the epidermis.皮的外层叫表皮。
• There is a neoplasm originating in his leg's epidermis.他的腿上有个生长在表皮上的肿瘤。

2 epidermal   

adj. [解][生]表皮的，外皮的

参考例句：

• Epidermal growth factor is a single-chain peptide with 53 amino-acid residue. 表皮生长因子(epidermal growth factor EGF)是最初从小鼠的下颌下腺提取的一种含有53个氨基酸残基的单链多肽。
• Telia are dark brown or black, and shine from the intact host epidermal covering. 冬孢子堆呈暗褐色或黑色，覆盖在完整的寄生表皮下显得发亮。

3 visualize   

vt.使看得见，使具体化，想象，设想

参考例句：

• I remember meeting the man before but I can't visualize him.我记得以前见过那个人，但他的样子我想不起来了。
• She couldn't visualize flying through space.她无法想像在太空中飞行的景象。

4 poked   

v.伸出( poke的过去式和过去分词 )；戳出；拨弄；与（某人）性交

参考例句：

• She poked him in the ribs with her elbow. 她用胳膊肘顶他的肋部。
• His elbow poked out through his torn shirt sleeve. 他的胳膊从衬衫的破袖子中露了出来。 来自《简明英汉词典》

5 strand   

vt.使（船）搁浅，使（某人）困于（某地）

参考例句：

• She tucked a loose strand of hair behind her ears.她把一缕散发夹到了耳后。
• The climbers had been stranded by a storm.登山者被暴风雨困住了。

6 pigment   

n.天然色素，干粉颜料

参考例句：

• The Romans used natural pigments on their fabrics and walls.古罗马人在织物和墙壁上使用天然颜料。
• Who thought he might know what the skin pigment phenomenon meant.他自认为可能知道皮肤色素出现这种现象到底是怎么回事。

7 technically   

adv.专门地，技术上地

参考例句：

• Technically it is the most advanced equipment ever.从技术上说，这是最先进的设备。
• The tomato is technically a fruit,although it is eaten as a vegetable.严格地说，西红柿是一种水果，尽管它是当作蔬菜吃的。