3. Toward the close of pregnancy, the development of the buttocks and the lower limbs is such that the pelvic becomes larger than the ce phalic extremity, and the fcetus changes its position, and remains head downward. Hence the frequency of vertex presentations at term.
4. These changes explain the active movements made by the fcetus during pregnancy.
" This position of the infant," says Bailly, " is the mechanical result of the shapes of the contained body and of its envelope, and is doubtless accomplished by the contractions of the uterus and the shaking of the fcetus by the motions of the mother." Pinard says: " It is now universally admitted that in the first six months of pregnancy the fundus, or superior segment of the uterus, is more developed than the inferior segment, and that the head is at that time the largest part of the fcetus. During the entire period of pregnancy painless uterine contractions occur; and it is proven that, when the uterus contracts, it diminishes its transverse and increases its longitudinal diam eters, and as no woman remains motionless during gestation, the movements of the mother must react more or less upon the fcetus." Martel goes still further: " The accommodation," he says, " must have been difficult; there must have been some special obstacle, when the vertex does not come in contact with the superior strait." He believes that, at the end of gestation, the fcetus becomes much less mobile, and remains in the position given it by the contractile sac in which it is lodged. He finds the cause of the folding of the fcetal body, and of its fixation, in the diminution of the amount of liquor amnii as compared with the volume of the fcetus.
Statistics all confirm the law of accommodation as stated by Pajot. Pinard, who, like all French accoucheurs, admits this law, regards the abdominal wall as the main factor rather than that of the uterus. While we do not deny that the contraction of the abdominal parietes does have a certain influence, it must be much inferior to the contractions of the uterine muscle itself. For Herrgott has demonstrated that the shape of the uterus has great influence upon the presentation, and that shape does certainly not depend exclusively upon the support offered to the organ by the abdominal parieties. There are certainly other elements in the question, though Pinard makes light of them. Wombs vary in thickness and in tonicity, especially after repeated childbirths. For if we find in multiparfe a larger proportion of presentations other than vertex than we do in primiparfe, it is not only because the abdominal walls are fiat and relaxed, but because the uterus has lost its tonicity and resisting power. Is it likely that the abdominal wall, so thin in certain women, and sepa rated from the foetus by both uterus and liquor amnii, should have a greater action on the foetus than the walls of the womb? As Martel says, how can we admit that relaxation of the abdominal wall will in any way affect the interior of the uterus, unless there is also relaxation of the uterine wall itself. The uterus might fall forward, but the change would
be merely one of position; the organ would still preserve the shape of an ovoid, with the smaller extremity directed downwards.
If, in primiparre, the head engages more often than in multiparre, it is not because, as Pinard says, the inferior segment of the uterus, the uterine diaphragm as he calls it, descends with the head into the pelvic cavity in the one case, and in the other case remains above it. No I The inferior segment of the uterus descends as much in the one case as in the other. If the head engages in primiparfe, while it does not do so in multiparty, it is because the laxity of the abdominal walls in the latter renders the accommodation of foetus to uterus less complete and less in timate.
This can be proved by ordinary observation. When primiparfe arrive at the end of their pregnancy, we find the head engaged and intimately applied to the inferior segment of the uterus, which is so thinned by the pressure of the head, and so moulded over it, that we can feel the sutures and fontanelles distinctly through it, and if we attempt to displace the head, we will displace the uterus together with it. In multiparw, on the contrary, the contact is never so intimate; the head is always, even when engaged, a certain distance above the inferior segment; it is rela tively mobile and can be lifted, and we can feel it recede before the finger and leave the inferior segment of the uterus. The head may be engaged, the presentation does not change; but the moulding of the uterine parietes to the head is less intimate, and the accommodation is 1?ss perfect. According to our ideas the abdominal wall plays but a secondary part in the process.
How, unless we take into account the peculiar shape of the uterus, and the special elasticity of its walls, can we account for those cases in which we see women in successive pregnancies, from the first to the last, have a series of breech or trunk presentations? We therefore entirely agree with the following conclusions which Martel has enunciated in his thesis: " If engagement occurs earlier in primiparfe, it is because of the greater rigidity of the uterine parietes, which keeps the foetal ovoid rigidly in the uterine axis. There is thus a tendency to the projection of the inferior part of the uterine ovoid into that part of the pelvic cavity sufficiently open to receive it. This downward progression is greatly favored by the flexion of the head, which is marked in most cases, though not complete, at the superior strait. Thus the head presents its smallest diameter at the superior strait, and passes through it prematurely.
" In multipara?, on the other hand, the uterine parietes are more dis tensible, the cavity is larger, the Rails is more mobile, and consequently the pressure is not exerted perpendicularly to the axis of the superior strait, but to a point behind the pelvis, or towards the abdominal walls, or even to the symphysis pubis. Therefore engagement does not so readily take place."