请问应该怎么理解what remained of a bottle of medicine从句
是新四里的一课：he went off immediately to visit him,carrying with him in his pocket 【what remained of a bottle of medicine】 formerly prescribed for an indisposition of Mrs.Carlyle's.
括号中由疑问词What引导的从句和一般的“疑问词+主语+谓语动词+句子剩余部分”的从句不同.如：Carlyle was entirely ignorant of what the bottle in his pocket contained.
这种从句应该怎样分析、理解?并请给我几个例句.