But a third stage of developtnent of the uterus is produced normally by the stimulus of impregnation, and partly- by the growth of the ovum, and abnormally by the formation of anv substance within the uterus, such as a poiypus, which may cause distension of its walls ; or by the accumulation of fluid in its cavity, such as the menstrual fluid collected in cases of atresia or imperforation of the vagina.
The developtnent of the uterus which is occasioned by the stimulus of pregnancy, takes place whether the impregnated ovum arrives within the uterine cavity or not ; although this does not occur in equal degrees in the two cases. In the case of extra-uterine preg nancy, a very considerable thickening of the uterine substance usually takes place, together with a general enlargement of the entire organ, fully equal to that which is observed in the third month, and, in some cases, when gestation is not interrupted, even in the fourth month of ordinary pregnancy.
In cases where gestation follows an ordi nary course, the development of the uterus is such, that the weight, at the end of the period, is found to he increased about twenty-four fold, and its length about five-fold.
This development, as it affects the size, weight, form, and position of the entire organ, as well as the physical condition of its special parts, will now be considered.
There is no example in man, and few in the animal kingdom generally, of a development of any organ or structure comparable in rapi dity with that which takes place in the uterus during gestation, although the periodical growth of the deer's horn, and the formation of the placenta, may be quoted as in some respects analogous cases.
Size.— The rate of increase of the uterus, during pregnancy, is subject to great varia tions. But, with due allowance for these, which are dependent chiefly upon the size of the fcetus and placenta, the quantity of liquor amnii, or the number of ova fertilised, an approximate estimate may be forrned of the average alterations in size and bulk which the organ exhibits at different periods of normal gestation.
These may be expressed in calendar months as follows : — minimum weight was 2lbs., and the weight relatively to the unimpregnated organ, was as 24 to 1.* Form.— The form of the uterus undergoes many changes in the course of gestation.
During the first three months, although there is a considerable increase of size, the primitive figure is retained with only slight alterations. After the third month, the body rapidly en larging, while the cervix remains nearly un altered, the figure of the former approaches that of a sphere. For the perpendicular and transverse diameters of the body then become nearly equal, and the only- deviation from the spherical forni is occasioned, first, by the cervix, which increases the vertical dia meter of the entire organ by one inch ; and secondly, by the more tardy expansion of the body in the antero-posterior diameter, pro ducing the form of a flattened sphere. After this, the perpendicular increasing more rapidly than the transverse diameter, and the upper segments widening faster than the lower ones, the uterus gradually acquires the ovoid figure which characterises it at the end of pregnancy.
Alterations, nearly corresponding with these, take place in the cavity of the uterine body. The walls of this flattened triangular chamber begin to separate from each other ; and by their gradual expansion, the angles and supe rior and lateral lines, by which the cavity was at first bounded, are unfolded, so that the tri angular is gradually exchanged for the pyri form shape, and this again for the figure of a flattened sphere —as in the fourth and fifth months of' gestation ; after which period the figure of the cavity corresponds very accu rately with the general external form of the organ.
During these alterations, the fundus be comes strongly arched; while the sides un dergo a slighter relative expansion, so that they exhibit only a gentle swelling ; but the anterior and posterior walls become curved and proniinent — sometimes the former, and sometimes the latter, according to Dr. W. Hunter, showing the greater amount of con vexity.t It has often been asked whether, during these changes, the walls of the uterus increase in thickness, or the contrary. In other words, whether the dilatation of the uterine cavity is to be regarded as a mere passive distension, with thinning of the walls ; or whether the process of enlargement consists of an active excentric hypertrophy.