Definition: The cervix is a small tubular structure connecting the uterus to the vagina. The internal os (opening in the cervix) connects the cervix to the uterus, and the external os connects the cervix to the vagina. The cervix protects the uterus from infection and facilitates the passage of sperm into the uterus. For most of the month, the external os is covered with a thick, sticky mucus, which is inhospitable to bacteria. Around the time of ovulation, the mucus thins out and forms watery strands, called spinnbarkeit, which make it easier for sperm to enter the uterus.