A microscopic examination may sometimes disclose the origin of the blood because of the presence of foreign particles. Mucous or hairs from the nostrils may be found in blood from the nose. Semen and genital hairs may be found in blood resulting from a sexual assault. Certain cells from the vaginal may be noted in blood from menstruation. Although the presence of foreign elements or particles may lead to conclusions as to the place of origin of the blood, their absence does not necessarily disprove the fact that the blood originated in the part of the body from which it was believed to have come.