This stage corresponds with that in which the young of oviparous animals breaks the shell and escapes. The human being however, undergoes a remarkable change, and remains in the womb for a period longer than that already past, in order to become more perfected.
From four to nine months the development is proportionally much more rapid than during the first four months, owing to the circulation of perfect red blood, which is now found the same as in the adult, and is probably derived from the mother's blood vessels.
Five months. Every part is considerably increased in size, and become more perfect. The lungs enlarge, and are even capable of being, to a certain extent, dilated. The skin becomes much stronger. The situation of the nails can be discerned. The meconium is more abundant, and lower down in the intestines. The length is now eight or ten inches, and the weight fifteen or sixteen ounces.
Six months. The nails are marked. The head becomes downy, from the first development of the hair. A little fat is formed. Length twelve inches, weight from one and a half to two pounds. No indications of intellectual faculties.
Seven months. The whole being has rapidly progressed. The nails are formed, the hair is perfect, in the male the testicles descend to the scrotum, and in the female the ovaries reach the brim of the pelvis. The bones are tolerably firm, and the meconium collects in the large intestines. Length fourteen inches, weight about three pounds. Intellectual functions not yet exercised.
The two remaining months are merely devoted to further increase in size and weight. No new phenomena present themselves.
Nine months. Every function has become active. The skin becomes colored, and perspiration occurs. There are no indications of the intellectual functions, but the animal functions are remarkably active, particularly that of taste, which no doubt leads to the act of sucking, from the natural desire for its gratification. The child can now experience all the ordinary sensations of pain, hunger, heat, and cold, and is capable of preserving an independent existence if brought into the world.
Plate XII. represents a section of the Uterus at about one month of gestation, so as to show all the parts in their proper situation.
PLATE XII.
Section of the Uterus, with the Ovum and appendages, at about one month of gestation.—a. a. a. The substance of the walls of the Womb.—b. b. The Embryo.—c. The different vessels by which it is connected with the Placenta.—d. d. The Placenta.—e. The Vitellus.—f. f. f. The Membrane lining the Uterus, called the Decidua; it is seen to be bent double, or reflected, the Embryo being on the outside of it.—g. g. The Chorion, or Middle Membrane, which is studded over with villosities, or small blood vessels.—h. h. The Amnion, or inner membrane, which contains the fluid called the liquor amnii, in which the Embryo floats.—i. i. The blood vessels which connect the Placenta with the Womb.—j. Is a plug of Mucus, by which the mouth of the Womb is now blocked up.—k. k. The ends of the Fallopian Tubes, which are cut off; these are also blocked up with mucus, the same as the Os Tincæ.—l. The Os Tincæ, or mouth of the Womb.—m. The Vagina.
Plate XII.
Section of the Uterus, with the Ovum and appendages, at about one month of gestation.
FŒTAL NUTRITION.
The manner in which the new being derives its nutriment, or the material by which it grows, is, in a great measure, unknown to us, though we certainly obtain some little information about it by a study of the apparatus employed in the process.
For the first fifteen or twenty days the substance called the Vitellus, (e. Plate XII.) which is analagous to the yelk of the ordinary egg, appears to supply most, if not all of the material that is required in the formation of the new being; and indeed this substance does not totally disappear till after the third month, though we cannot suppose it to be the sole source of nutriment then. It is also supposed, by some, that the amniotic liquor, in which the fœtus floats, may afford some nutriment, either by being swallowed, or by being absorbed through the skin. It is certain that this fluid is nutritive, and there is nothing impossible in its absorption, though it is not very likely to occur to a sufficient extent. The idea that it can be swallowed however, is erroneous, because the mouth of the Fœtus is firmly closed while in the Womb; and besides, children have been born alive without mouths, and even without heads, and of course they could not have swallowed anything. It is now generally conceded by physiologists that the material required by the Fœtus, for its nutrition, is obtained from the blood of the mother, through the medium of the Placenta, and the vessels in the Umbilical cord. It is, however, a matter of dispute whether the maternal blood is sent directly, in its ordinary state, into the body of the child, or whether it first undergoes a preparatory process, which most modern authors suppose it does.
From the earliest period of gestation, the middle membrane, called the chorion, (g. g. P. XII.) is covered, on its outer surface, with a number of small protuberances called villosities, which subsequently become true blood vessels. About the fourth month these have increased very much in size and number, and have all become conglomerated into one mass, in form like a mushroom. This is called the Placenta. It is almost entirely formed of blood vessels, which seem to attach themselves at one end, by open mouths, to the open mouths of other blood vessels on the inner walls of the uterus (i. i. Plate XII.) At the other end these vessels are drawn together and lengthened out into a long tube, called the umbilical cord, or navel string, which finally enters the body of the child at the navel and so establishes the connexion between it and the mother.—(c. Plate XII.)
The blood vessels in the placenta, umbilicus, and fœtus, like those in the maternal body, are of two kinds, Arteries and veins. The arteries, which come from the left side of the heart, carry the pure blood, which contains all the materials for forming and nourishing every part of the system. The veins contain the blood in its impure state, and take it to the right side of the heart, from whence it is forced into the lungs to be purified by the act of breathing. The blood is made impure by some of its constituents being absorbed, to form the different parts of the body, and by having thrown into it a quantity of waste and poisonous matter no longer needed.
The course of the blood, therefore, is from the left side of the mother's heart along her arteries till it reaches the arteries of the uterus, from them it passes into those of the placenta, and thence into those of the umbilicus which convey it into the body of the child. When there it circulates in its arteries, supplies the material for its further increase and development, becomes in consequence impure, and passes into its veins, the same as in the maternal body. From these veins it passes into those of the umbilicus and placenta, and, apparently, into those of the mother, by which it is conveyed to the right side of her heart, and by its action to her lungs, to be again purified when she breathes. This explains what was previously stated, that the child uses the mother's heart, lungs, and stomach, while in the womb, and has, therefore, no occasion to use its own.
The diameter of the placenta is about six inches, and its thickness about one inch and a half. The length of the umbilical cord is from eighteen to twenty-four inches, its diameter about half an inch. These dimensions are, however, subject to great variation. Instances are mentioned of the cord being five feet long, and as thick as the child's