Four months after conception the length of the fœtus is six or seven inches; every part of its body is so greatly augmented as to be perfectly distinguished from each other; even the nails appear on the fingers and toes. The testicles of the males are shut up in the belly above the kidneys; the stomach is filled with somewhat of a thick humour, like that which incloses the amnios. We find a milky fluid in the little vessels, and in the large ones a black liquid matter. There is a little bile in the gall, and some urine in the bladder. As the fœtus floats freely in the liquid which surrounds it, there is always a space between the body and membranes in which it is contained. These coverings grow at first more than the fœtus; but after a certain time it is quite the contrary. Before the end of the third month the head is bent forward, the chin rests on the breast, the knees are lifted up, the legs bent backwards upon the thighs (sometimes the knees are so high as almost to touch the jaws), the arms are generally folded across the breast, and one of the hands, and often both touch the face. The fœtus afterwards takes different situations, as it acquires strength. Experienced midwives have pretended to be certain that it changes much oftener than is commonly thought, and which they prove by several observations; first, the umbilical cord is often found twisted round the body and limbs of the child, in a manner which necessarily supposes, that the fœtus has moved in many directions, and taken different positions; secondly, a mother feels the motions of the fœtus sometimes on one side of the womb and sometimes on another; and it often strikes against many different places, which must be occasioned by different positions, and supposes that it takes different situations; thirdly, as it floats in a liquid which surrounds it on all sides, it can very easily turn and extend itself by its own strength; and it must also take different situations according to the various attitudes of the mother; for example, when she lies down, the fœtus must be in another situation to what it was when she stood upright.
Most anatomists have said, that the fœtus is constrained to bend its body, because it is too confined in its covering; but this opinion does not appear well founded, for in the first five or six months there is more space than is required for the fœtus to extend, and yet during that time it is bent and folded. We also see the chicken is in a curved posture in the liquor of the amnios, although this membrane and its liquor are sufficient to contain a body five or six times as large as the fœtus. Thus we may conclude that this curved form of the fœtus is natural, and not the effect of force. I am somewhat of Harvey's opinion, who says, it takes this attitude because it is the most favourable to rest and sleep; and as the fœtus sleeps almost continually, it naturally takes the most advantageous situation. "Certe (says this famous anatomist) animalia omnia, dum quiescunt & dormiunt, membra sua ut plurimum adducunt & complicant, figuramque ovalem ac conglobatam quærunt: ita pariter embryones qui ætatem suam maxime somno transigunt, membra sua positione ea qua plasmantur (tanquam naturalissima ac maxime indolenti quietique aptissima) componunt[AE]."
[AE] Harvey on Generation, page 257.
The matrix, as we have already said, takes a very ready growth after conception, and it continues also to increase in proportion with the fœtus; but the fœtus at length outgrows the matrix, and then, especially when it approaches maturity, it may be too much confined, and agitate the matrix by reiterated motions and violent efforts. The mother sensibly feels the impression of these painful sensations, and which are called periodic pains after the labour commences. The more power the fœtus exerts to dilate the matrix the greater it finds the resistance, from the natural compression of the parts. From thence all the effect falls on the orifice, which has been increasing by degrees during the latter months of pregnancy. The head of the fœtus, forcibly inclining against the sides of the orifice, dilates it, by a continual pressure, till the moment of delivery, when it opens sufficiently for the child to escape from the womb.
What makes it probable that the labour-pains proceed only from the dilatation of the orifice of the matrix is, that this dilatation is the only means to discover whether the pains felt are in fact the pains of labour, for women often feel very sensible pains, which are not those that immediately precede delivery. To distinguish the false from true pains, it has been recommended for the midwife to touch the orifice of the matrix, as if the pains be true the dilatation will always increase, and if they are false pains, that is to say, pains which proceed from some other cause than that of the approaching delivery, the orifice will contract rather than dilate, or at least will not continue to dilate. From hence we have sufficient foundation to imagine, that these pains proceed from a forced dilatation of the orifice. The only thing which embarrasses on this occasion is that alternative of rest and sufferings the mother endures. This circumstance of the effect does not perfectly agree with the cause which we have just indicated; for the dilatation of an orifice, which is made by degrees, should produce a constant and continued pain, without any intervals of ease. But possibly the whole may be attributed to the separation of the placenta, which we know is fastened to the matrix by a number of papillæ, which penetrate into the vacuities or cavities of this viscera; therefore may it not be supposed that they do not separate from their cavities all at the same time; that each separation causes those acute pains, and the intervals between are those of ease and rest? The effect in this case perfectly answers the cause, and we can support this conjecture by another observation.—Immediately before delivery there issues a whitish and viscous liquor, like that which flows from the nipples of the placenta when drawn out of their places, which makes it probable that this liquor, which then issues from the matrix, is produced by the separation of some of the papillæ of the placenta.
It often happens that the fœtus quits the matrix without bursting the membranes, and consequently without the contained liquor flowing out. This kind of delivery appears to be most natural, and resembles that of most animals; nevertheless, the human fœtus commonly pierces its membranes by the resistance it meets with at the orifice of the matrix. It also sometimes brings away part of the amnios, and even the chorion, upon its head like a cap. When these membranes are pierced or torn, the liquors, called the waters, which they contain flow out, and the sides of the orifice of the matrix, and the vagina, being thus moistened, give way more easily to the passage of the child. After the flowing of this liquor there remains sufficient room in the matrix for the midwife to return the child, if the position is unfavourable. When the fœtus is come out the delivery is not entirely completed, the placenta and membranes remain in the matrix, and the new-born infant adheres to them by the umbilical cord; the hand of the midwife, or the weight of the body of the infant alone, draws them out by means of this cord. Those organs which were necessary to the life of the fœtus become useless, and even noxious to the new-born infant. They are instantly separated from the body of the child, by tying the umbilical cord about an inch distance from the navel, and by cutting it about an inch from the ligature. The remainder of this cord dries away, and separates of itself from the navel, about the sixth or seventh day.
On examining the fœtus previous to its birth we may form some idea of its natural functions. It has organs, which are necessary to it while in the womb of its mother, but which become useless. For the better understanding the mechanism of these functions, we must explain a little more particularly the nature of those necessary parts, the umbilical cord, the membranes, the liquor which they contain and the placenta. The umbilical cord, which is attached to the body of the fœtus at the navel, is composed of two arteries and one vein; these prolong the circulation of the blood, but the vein is larger than the arteries. At the extremity of the cord each of these vessels divide into an infinity of ramifications, which extend between two membranes. They separate at equal distances from the common trunk; so that these ramifications are round and flat, and are called, when thus collected, the placenta. The external surface, which is applied against the matrix, is convex; the internal concave. The blood of the fœtus circulates in the cord, and in the placenta. The arteries of the cord spring from two large arteries of the fœtus, and carry the blood through the arterial ramifications of the placenta; from thence it passes into the venous branches which carry it into the umbilical vessels; these communicate with a vein of the fœtus, in which vessels it is received.
The concave surface of the placenta is clothed by the chorion; the convex is also covered by a kind of soft membrane, easily torn, which seems to be a continuation of the chorion, and the fœtus is included under the double coat of the chorion and the amnios. The form is globular, because the intervals between the membranes and the fœtus are filled with a transparent liquor. This liquor is contained by the amnios, which is the internal membrane, it is thin and transparent; it folds round the umbilical cord at its insertion into the placenta, and covers it the whole length to the navel of the fœtus. The chorion is the external membrane; it is thick and spongy, sprinkled with sanguinary vessels, and composed of many coats, the exterior of which covers the convex surface of the placenta. It follows the inequalities, and covers the papillæ, which spring from the placenta, and are received in the cavities found at the bottom of the matrix, called lacunæ. The fœtus adheres to the matrix by these insertions.
Some anatomists have thought that the human form had, like those of certain quadrupeds; a membrane called allantois, destined to receive the urine; and they have pretended to have found it between the chorion and the amnios, or in the middle of the placenta at the root of the umbilical cord, under the form of a very large bladder, in which the urine entered by a long pipe that composed part of the chord, and which opened on one side into the bladder, and on the other in this allantois membrane, being similar to the urachus in other animals. They owned, however, that it was not near so large in the human fœtus as in quadrupeds, but that it was divided into many tubes, so minute, that they could scarcely be perceived, and that the urine passed into their cavities.