Sphincter of the vagina

Sphincters are areas of smooth muscle that form a specific ring. In humans there are about six of them. One of them, the bladder sphincter, is located in the urethra and can also be seen when urinating. But the internal anal and external uterine rings - prolapsing walls of internal hollow organs (mainly the digestive tract and genital tract), which are capable of turning outward, were discovered only in the 19th century.