In interstitial cases the walls of the sac surrounding the ovum sometimes attain in parts a thickness nearly equal to that of the gravid uterus. On section of these portions the appearance which they present is precisely shnilar to that of the gravid uterus itself. There is here seen precisely the same arrange ment of large vascular openings, being the divided canals or sinuses which everywhere permeate the solid walls, in whose composi tion may be traced the same abundance of smooth muscular fibre, as in the ordinary gravid uterus.
Within such a sac, formed out of the walls of the tube in the first instance, and in the case of this third variety further strengthened by the addition of a large quantity of tissue derived from the uterus, the ovura lies, pre senting its ordinary character of an external chorion and inner amnion ; the foetus or embryo itself, according to the period of ges tation, being perfectly formed. The walls of the sac, being in this case usually much stronger than when the ovum lies nearer to the distal end of the tube, resist pressure for a longer time, and consequently the foetus may attain a greater growth.
One of the most interesting questions con nected with this subject is, whether a decidua is here formed. Schroeder van der Kolk, in his recent most valuable work on the struc ture of the Placental-, answers the inquiry in the affirmative, in contradiction to the state ment of Virchow t, by whom it is asserted that in the case of tubal gestation no decidua is to be found in the tube. According to Schrceder, a decidua is here formed in tubal pregnancy, notwithstanding that in the walls of the tube glandulw utriculares are entirely wanting. The villi are here embedded in little hollows of the decidua, upon whose walls the blood vessels terminate in open mouths, and thus the blood is poured out into the placenta. The decidua is, indeed, in this case firmer, and does not exhibit so many valvular openings as are present in an ordi nary placenta ; probably from the absence of the utricular glands. In this case, also, an epithelial layer derived from the decidua covers the and serves at the same time as a means of junction between the parts.* Associated usually with the abnormal deve lopment of the ovum in the oviduct is the for mation of a decidua in the uterus, the nature of which structure will be considered in a subse quent portion of this article (pp. 635. 652).
And here it naturally occurs to inquire into the probable causes of the development of the ovum in a situation so unfavourable to its further and complete evolution. Since, not withstanding the wonderful power of adapta tion which is in these cases exhibited by the parts immediately surrounding and containing the ovum, it is plain that the oviduct how ever altered, yet, on account of its peculiar form and texture, can but inadequately supply the offices of a uterus. It can serve but im perfectly for the nutrition and protection of the fcetus, and not at all for its expulsion, even should the latter reach the term of its dependent or intra-uterine life.
One of the most remarkable circumstances relating to this curious subject, is the fact first noticed, I believe, by Dr. Oldham, that in a large number of cases of tubal gestation, the corpus luteum, corresponding vvith the ovum impregnated, is found in the ovary of the op posite side to that of the tube in which the ovum is developed. Thus if the left Fallo pian tube contains the ovum, the right ovary will often display the corpus luteum of a cor responding date, and vice versii. Not being at first aware of Dr. Oldham's observation, I had myself noticed the same circumstance in re peated instances, and had arrived at the same conclusion as he has done in explanation of it, namely, that at the time of the ovum quitting the ovary, the tube of the one side embraced the opposite ovary, and conducted away the ovum, which being impregnated in the ordinary way, and then being delayed at the angle formed by the bending of the tube, has its further progress obstructed at that point until it attains too great a size to ad mit of its subsequently passing the lower orifice and entering the cavity of the uterus.
If it be objected that this explanation is not satisfactory, because it assumes the ap parent improbability of the fimbriated ex tremity of one Fallopian tube being able to grasp the opposite ovary, then I can point to a preparation in the Cambridge University Anatomical Museum *, in vvhich both the Fallopian tubes grasp the same ovary to which their extremities are affixed by morbid ad hesion.