Uterine portion of the tube, pars uterina. — This, as just stated, is the portion of the ovi duct which traverses and is included in the substance of the uterine walls. Its length will vary, in some degree, with the varying thickness of those walls, in different subjects ; yet not entirely so, because this canal does not pierce the uterine parietes in a direction perpendicular to their surface, but traverses them in an oblique manner, while the tissues become gradually attenuated around it, in a direction from within outwards (fig. 405. b).
But the course of the tube through the uterine walls• may be still more satisfactorily traced by the aid of a section made down to, but not laying open, the canal. The peculiar white colour of the tube is thus made to con trast strongly with the surrounding darker uterine tissue ; and this peculiarity is ren dered more striking when a fine injection of the part has been made. The canal of the tube may thus be readily traced from its infundibular-shaped commencement running, in the first half of its course, in a direction obliquely npwards and outwards, and in its remaining half, either horizontally outwards, or more commonly turning rather suddenly downwards, and forming, with its first rection, an angle of 60° (fig. 406. and 431.).
Strictly, the Fallopian tube should be deemed to commence at this point ; and this should be regarded as the true odium uterinum, while the short infundibular canal leading to it from the uterine cavity should be con sidered a portion of that cavity, representing, in fact, the cornu of the uterus in mammalia. The peculiar form of this portion of the tube is not without interest, for it appears to me to offer a probable explanation of the occa sional detention of the iinpregnated ovum, in its passage through this division of the ovi duct, where its development produces the variety of extra-uterine pregnancy termed by Breschet interstitial.
Canal of the body of the tube. — While the portion of the Fallopian tube already de scribed, as contained within the substance of uterine walls, is rightly termed its uterine or fixed portion, the main part, which is ex ternal to them, constitutes the free portion. This also is traversed in its entire length by a canal, the form of which corresponds generally with that of the tube itself It is occupied by numerous longitudinal folds of the lining membrane (fig. 405. c), which are
so closely placed as to convert the channel of the tube into a series of minute capillary canals. These folds never disappear by dis tension like the folds and furrows upon many mucous surfaces, such as the cesophagus, blad der, &c. ; but they are true plications, like the valvulre conniventes of the small intes tine, as pointed out by M. Richard, who has very accurately described their arrange ment.* Each of these is composed of two layers of mucous membrane united together by cellular tissue. Their direction is con stantly parallel with the axis of the tube. In the uterine region of the oviduct, they con stitute two or three small projecting and rigid crests, forming the little capillary chan nels, but in proportion as they advance to wards the outer part, they become more elevated and numerous, and at 2 or 3 fingers' breadth from the uterus commence the large floating folds which are prolonged as far as the pavilion. These floating plaits are from 4 to 6 in number ; they acquire a breadth of 2-3"', and are themselves co vered by an infinite number of little crests, often imbricated the one npon the other, and intercepting between them little capil lary canals. On a level with the abdomi nal opening these large folds cease, the small ones only remaining ; but still one of these large folds always extends beyond the ori fice.
External orifice, ostium abdominale. — This occupies the bottom of the funnel-shaped expansion or trumpet-like end of the ovi duct, and is formed simply by a constriction of the tubal vvalls at a short distance from the irregularly notched margin in which they terminate. The aperture is fringed in its entire circumference by the plications of the membrane already described (fig. 405.). These radiating towards the centre appear nearly to obstruct the entrance of the tube, which, however, during the middle period of life is usually of sufficient capacity to admit easily of the introduction of a moderate sized catheter. The constriction which forms this aperture is not occasioned by any thickening nor other alteration of texture in the walls of the tube, so that after the parts have been laid open, it is often difficult to determine the exact seat of the previously existing orifice by any raark except that of a slight diminution in breadth of the walls at this spot.