practical necessity : 实际需求

needed to ...做effort的后置定语

接下来课文...This is what the sculptor must do. He must strive continually to think of , and use, form in its full spatial completeness.He gets the solid shape, as it were, inside his head--he thinks of it, whatever its size, as if he were holding it completely enclosed in the hollow of his hand. ...

strive to : 追求

solid 固定的=physical 实在的

inside his head 在他脑子中间

whatever its size=no matter what size it is 不论其什么样的形状

enclose : 包着 (做宾补)

接下来课文...He mentally visualizes a complex form from all round itself; ...

all round : 整个周围