Beneath the peritoneum of the gravid ute rus is always found a large development of strong fibrous tissue, arranged in irregular cords and bundles. These sub-peritoneal fibres serve to strengthen the coats, and pro bably greatly contribute to prevent rupture of the organ, especially during labour.
The muscular or 'middle tissues of which this coat is cotnposed, together with their 'node of arrangement in the unimpreg nated uterus, have been already fully de scribed. And it is to an increase of these, but especially of the vascular and muscular elements, that the enormous growth of the uterus during pregnancy is chiefly due. This growth consists partly in a greater develop ment of the already existing structures, and partly in new formations.
The growth of the contractile fibre cells is here of especial interest. The elements of this tissue have been shown to consist, from infancy onwards, of fusiform fibre cells, inter mixed uith the round, oval, and elongated nu clei tfig.434.),which constitute their embryonic condition. These, up to the time of impregna tion, form the special and sole elements of the muscular tissue ; yet some physiologists even of the present day refuse to recognise in these a muscular character, although it is plain that the uterus so constructed has a contractile power. The occurrence of ab ortion, sometimes at the very beginning of pregnancy, the expulsion of polypi and dys menorrhceal membranes, and the painful con tractions trrmed uterine colic, prove that the unimpregnated uterus is so endoIN ed. This non-recognition of a muscular character in the uterus before pregnancy has arisen from the minute size of the individual fibres, and from. the difficulty of explaining why these should grow to a given point, and then cease to be developed. But F. M. Kilian has given a happy illustration of this point, de rived from the observation of KbIliker, that the contractile fibre cells which are found in the coats of the smaller blood-vessels, pre serve a relative proportionate size to those of the larger ones, wherein they are more fully developed. So also the contractile fibre cells of the uterus proceed to a certain point of development in the unimpregnated organ, and there stop. And in this respect it makes
little or no difference whether the organ exa mined has been taken from an infant or an adult.
But when pregnancy takes place, the fibres proceed to a further stage of development. Their growth is now so considerable, that the contractile fibre cells, instead of a length of 0'002-0'003n and width of 0.002"', in the fifth month, present a length of 0.06-0•12'", and width of 0'0025-0'006m, or even 0.01"1, and in the second half of the sixth month, a length of 0'1-0•25'", a width of 0'004,— 0'005"c and a thickness of 0'002-0.0028"'; consequently their length is increased from seven to eleven times, and their width from twice to five times.% But in addition to this greater development • of pre-existing fibre cells, a new formation of muscular fibre also takes place. This is ob served, according to KbHiker, chiefly in the inner layers, although it may also occur in the external ones. The time of this new forma tion is chiefly the first half of pregnancy, the earlier forms of the fibre cells being no longer discernible after the twenty-sixth week. From this time onwards, the muscular coat contains only colossal fibre cells.
According to my observations, the indivi dual fibre cells increase gradually in breadth throughout pregnancy, but their length is so variable, that the measurements just given can only' be re,garded as examples. The length, indeed, of the greater number of fibre cells after the third month cannot be 4,termined with exactitude. A great many are thrown into numerous folds and contortions. Some exhibit transverse wrinkles, and the majority, when unbroken, end in long drawn out fila ments, whose terminations become inter mingled with the adjacent cells. Fine longi tudinal markings are often distinguishable, and some fibres exhibit an elongated nucleus. The interior of' the fibre is finely granular, and the margins show often a sinuous outline.
The fibrous tissue uniting the several layers of muscles appears also to increase considerably, and towards the end of preg nancy to exhibit a distinct fibrillation.