Interference with the passage of the ovum along the tube has been attributed to a variety of causes. Chronic salpingitis is a frequent cause. It destroys the cilia of the epithelial cells of the tubal mucosa. It produces thickening of the tubal walls, and causes peritoneal adhesions that impede the normal peristaltic action of the tube. Obstruction to the passage of the ovum may also be caused by polypi or tumors of the tube; by tumors external to the tube pressing upon it; by displacement and hernia of the tube; by diverticula of the tube; or by abnormal foldings of the tubal wall. Tubal pregnancy has occurred in tubes in which no lesions whatever could be discovered by the most careful examination. It seems probable that practically all pregnancies that occur outside of the uterus originate in the Fallopian tube. Pregnancy may occur in any part of the tube from the abdominal ostium to the uterus. Tubal pregnancy is said to be infundibular when gestation begins in the infundibulum or in an accessory tube-ending. This variety has also been called tubo-ovarian, because in time the gestation-sac may become adherent to the ovary and be bounded by both tube and ovary. [Illustration: FIG. 155.--Tubal pregnancy, removed before rupture. The opening that has been cut in the tube shows the chorionic villi.] The pregnancy is said to be ampullar when gestation begins in the ampulla of the tube. This is the most usual seat of tubal pregnancy. It is called interstitial when gestation begins in the interstitial portion, or that part of the tube in immediate relationship with the uterus. =Changes in the Fallopian Tube.=--During the early stages of tubal pregnancy--the first two or three months--it seems probable that a certain amount of hypertrophy and hyperplasia of the muscular wall of the tube takes place. The general form of the tube is spindle-shaped (Fig. 155). There is a marked increase in the vascularity of the tube, most pronounced in the neighborhood of the ovum. The whole tube becomes turgid and swollen. The peritoneal margin or ring surrounding the ostium abdominale becomes prominent, and gradually, as has already been described under Salpingitis, projects beyond the fimbriæ, contracts, and ultimately hermetically closes the ostium. Inflammation of the peritoneal covering of the tube may be present. Such inflammation may have preceded the tubal pregnancy or may have occurred as the result of the pregnancy. It produces various tubal adhesions and distortions, and may still more firmly close the abdominal ostium. The changes that take place in the mucous membrane of the tube and in the developing ovum are similar to those that occur in the uterus in a normal pregnancy. A variety of terminations occur in tubal pregnancy: I. In very exceptional cases the pregnancy may continue until full term, without rupture of the tube taking place.