Original: This is because a male cat 's penis has a band of about 120 â '' 150 backwards-pointing spines , which are about one millimeter long ; upon withdrawal of the penis , the spines rake the walls of the female 's vagina , which is a trigger for ovulation .
Simplified: This is because a male cat 's penis has a band of about 120-150 backwards-pointing spines , which are about one millimeter long ; upon withdrawal of the penis , the spines rake the walls of the female 's vagina , which is a trigger for ovulation .