Skunks apparently try to avoid getting ANY musk on their fur. This shows the 'retractable nipples' in action. A skunk cannot spray when held up by its tail. It appears to depend on the individual. If lifted after the nipples protrude, then a skunk can successfully spray.
Skunk spray can cause permanent blindness.
A skunk's musk consists of 1-Butanethiol. Many books which talk about skunks continue to use Aldritch's results. Two were minor variants. The following page describes this analysis of skunk spray.