俱舍論疏 12.10

12.10

「論曰至為指節」，已上十一位皆七成也。兔毛細滑塵小羊毛、牛毛潤膩塵大羊毛也。言七成者，六方并心以為七也。

On the words, "The Śāstra says: [...] forms a finger joint": The eleven ranks above are each formed by sets of seven. The hair of a hare is fine and smooth, so its dust is smaller than that of sheep hair; the hair of an ox is glossy and oily, so its dust is larger than that of sheep hair. To say "formed by sets of seven" means the six directions plus the centre make seven.